diff --git a/apps/web/app/(use-page-wrapper)/auth/error/page.tsx b/apps/web/app/(use-page-wrapper)/auth/error/page.tsx
index 48cc2ced27..6beb145de3 100644
--- a/apps/web/app/(use-page-wrapper)/auth/error/page.tsx
+++ b/apps/web/app/(use-page-wrapper)/auth/error/page.tsx
@@ -47,6 +47,8 @@ const ServerPage = async ({ searchParams }: PageProps) => {
           ? "SAML (like Okta)"
           : "your original login method";
       return t("account_managed_by_identity_provider_error", { provider: providerName });
+    } else if (error === "saml-idp-not-authoritative") {
+      return t("saml_idp_not_authoritative_error");
     }
     return t("error_during_login") + (error ? ` Error code: ${error}` : "");
   };
diff --git a/apps/web/public/static/locales/en/common.json b/apps/web/public/static/locales/en/common.json
index c0b49aecab..db5d653ab2 100644
--- a/apps/web/public/static/locales/en/common.json
+++ b/apps/web/public/static/locales/en/common.json
@@ -4168,6 +4168,7 @@
   "timestamp": "Timestamp",
   "json": "JSON",
   "hubspot_ignore_guests": "Do not create new records for guests added to the booking",
+  "saml_idp_not_authoritative_error": "This SAML identity provider is not authorized to manage your account. Please sign in with your original login method.",
   "audit_logs_organization_required": "You must be part of an organization to view audit logs.",
   "audit_logs_booking_not_found_or_permission_denied": "Booking not found or you do not have permission to view its audit logs.",
   "audit_logs_booking_has_no_owner": "Cannot verify permissions: booking has no associated user.",
diff --git a/packages/features/auth/lib/next-auth-options.ts b/packages/features/auth/lib/next-auth-options.ts
index ee195a4b49..4e0465bae3 100644
--- a/packages/features/auth/lib/next-auth-options.ts
+++ b/packages/features/auth/lib/next-auth-options.ts
@@ -47,6 +47,7 @@ import { teamMetadataSchema, userMetadata } from "@calcom/prisma/zod-utils";
 import { getOrgUsernameFromEmail } from "../signup/utils/getOrgUsernameFromEmail";
 import { ErrorCode } from "./ErrorCode";
 import { dub } from "./dub";
+import { validateSamlAccountConversion } from "./samlAccountLinking";
 import CalComAdapter from "./next-auth-custom-adapter";
 import { verifyPassword } from "./verifyPassword";
 
@@ -315,6 +316,10 @@ if (isSAMLLoginEnabled) {
       lastName?: string;
       email?: string;
       locale?: string;
+      requested?: {
+        tenant?: string;
+        product?: string;
+      };
     }) => {
       log.debug("BoxyHQ:profile", safeStringify({ profile }));
       const userRepo = new UserRepository(prisma);
@@ -329,6 +334,8 @@ if (isSAMLLoginEnabled) {
         name: `${profile.firstName || ""} ${profile.lastName || ""}`.trim(),
         email_verified: true,
         locale: profile.locale,
+        // Pass SAML tenant for domain authority checks in signIn callback
+        samlTenant: profile.requested?.tenant,
         ...(user ? { profile: user.allProfiles[0] } : {}),
       };
     },
@@ -939,6 +946,15 @@ export const getOptions = ({
             existingUserWithEmail.emailVerified &&
             existingUserWithEmail.identityProvider !== IdentityProvider.CAL
           ) {
+            // Verify SAML IdP is authoritative before auto-merge
+            if (idP === IdentityProvider.SAML) {
+              const samlTenant = (user as { samlTenant?: string }).samlTenant;
+              const validation = await validateSamlAccountConversion(samlTenant, user.email, "SelfHosted→SAML");
+              if (!validation.allowed) {
+                return validation.errorUrl;
+              }
+            }
+
             if (existingUserWithEmail.twoFactorEnabled) {
               return loginWithTotp(existingUserWithEmail.email);
             } else {
@@ -952,6 +968,15 @@ export const getOptions = ({
             !existingUserWithEmail.emailVerified &&
             !existingUserWithEmail.username
           ) {
+            // Verify SAML IdP is authoritative before claiming invited user
+            if (idP === IdentityProvider.SAML) {
+              const samlTenant = (user as { samlTenant?: string }).samlTenant;
+              const validation = await validateSamlAccountConversion(samlTenant, user.email, "Invite→SAML");
+              if (!validation.allowed) {
+                return validation.errorUrl;
+              }
+            }
+
             await prisma.user.update({
               where: {
                 email: existingUserWithEmail.email,
@@ -981,6 +1006,15 @@ export const getOptions = ({
             existingUserWithEmail.identityProvider === IdentityProvider.CAL &&
             (idP === IdentityProvider.GOOGLE || idP === IdentityProvider.SAML)
           ) {
+            // Verify SAML IdP is authoritative before converting account
+            if (idP === IdentityProvider.SAML) {
+              const samlTenant = (user as { samlTenant?: string }).samlTenant;
+              const validation = await validateSamlAccountConversion(samlTenant, user.email, "CAL→SAML");
+              if (!validation.allowed) {
+                return validation.errorUrl;
+              }
+            }
+
             await prisma.user.update({
               where: { email: existingUserWithEmail.email },
               // also update email to the IdP email
@@ -1002,6 +1036,13 @@ export const getOptions = ({
             existingUserWithEmail.identityProvider === IdentityProvider.GOOGLE &&
             idP === IdentityProvider.SAML
           ) {
+            // Verify SAML IdP is authoritative before converting account
+            const samlTenant = (user as { samlTenant?: string }).samlTenant;
+            const validation = await validateSamlAccountConversion(samlTenant, user.email, "Google→SAML");
+            if (!validation.allowed) {
+              return validation.errorUrl;
+            }
+
             await prisma.user.update({
               where: { email: existingUserWithEmail.email },
               // also update email to the IdP email
diff --git a/packages/features/auth/lib/samlAccountLinking.test.ts b/packages/features/auth/lib/samlAccountLinking.test.ts
new file mode 100644
index 0000000000..59cf64a478
--- /dev/null
+++ b/packages/features/auth/lib/samlAccountLinking.test.ts
@@ -0,0 +1,243 @@
+import type { PrismaClient } from "@prisma/client";
+import { describe, it, expect, vi, beforeEach, type MockInstance } from "vitest";
+
+import { MembershipRepository } from "@calcom/features/membership/repositories/MembershipRepository";
+import { OrganizationSettingsRepository } from "@calcom/features/organizations/repositories/OrganizationSettingsRepository";
+
+import {
+  SamlAccountLinkingService,
+  getTeamIdFromSamlTenant,
+  validateSamlAccountConversion,
+} from "./samlAccountLinking";
+
+vi.mock("@calcom/lib/logger", () => ({
+  default: {
+    getSubLogger: () => ({
+      warn: vi.fn(),
+      error: vi.fn(),
+      debug: vi.fn(),
+    }),
+  },
+}));
+
+vi.mock("@calcom/prisma", () => ({
+  prisma: {},
+}));
+
+const mockPrismaClient = {} as PrismaClient;
+
+let hasAcceptedMembershipSpy: MockInstance;
+let getVerifiedDomainsSpy: MockInstance;
+
+function setupMocks(config: { verifiedDomains?: string[]; hasMembership?: boolean }) {
+  hasAcceptedMembershipSpy = vi
+    .spyOn(MembershipRepository.prototype, "hasAcceptedMembershipByEmail")
+    .mockResolvedValue(config.hasMembership ?? false);
+
+  getVerifiedDomainsSpy = vi
+    .spyOn(OrganizationSettingsRepository.prototype, "getVerifiedDomains")
+    .mockResolvedValue(config.verifiedDomains ?? []);
+}
+
+describe("getTeamIdFromSamlTenant", () => {
+  it("extracts team ID from valid tenant string", () => {
+    expect(getTeamIdFromSamlTenant("team-123")).toBe(123);
+    expect(getTeamIdFromSamlTenant("team-1")).toBe(1);
+    expect(getTeamIdFromSamlTenant("team-999999")).toBe(999999);
+  });
+
+  it("returns null for invalid tenant formats", () => {
+    expect(getTeamIdFromSamlTenant("")).toBeNull();
+    expect(getTeamIdFromSamlTenant("invalid")).toBeNull();
+    expect(getTeamIdFromSamlTenant("org-123")).toBeNull();
+    expect(getTeamIdFromSamlTenant("team-")).toBeNull();
+    expect(getTeamIdFromSamlTenant("team-abc")).toBeNull();
+  });
+
+  it("returns null for tenant with non-numeric ID", () => {
+    expect(getTeamIdFromSamlTenant("team-abc123")).toBeNull();
+  });
+
+  it("truncates decimal values (parseInt behavior)", () => {
+    expect(getTeamIdFromSamlTenant("team-12.5")).toBe(12);
+  });
+});
+
+describe("SamlAccountLinkingService.isSamlIdpAuthoritativeForEmail", () => {
+  let service: SamlAccountLinkingService;
+  const ORG_TEAM_ID = 123;
+
+  beforeEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("returns authoritative when email domain matches org verified domain", async () => {
+    setupMocks({ verifiedDomains: ["acme.com"], hasMembership: false });
+    service = new SamlAccountLinkingService(mockPrismaClient);
+
+    const result = await service.isSamlIdpAuthoritativeForEmail(ORG_TEAM_ID, "user@acme.com");
+
+    expect(result).toEqual({ authoritative: true, reason: "domain_verified" });
+    expect(hasAcceptedMembershipSpy).not.toHaveBeenCalled();
+  });
+
+  it("performs case-insensitive domain matching", async () => {
+    setupMocks({ verifiedDomains: ["acme.com"] });
+    service = new SamlAccountLinkingService(mockPrismaClient);
+
+    const result = await service.isSamlIdpAuthoritativeForEmail(ORG_TEAM_ID, "user@ACME.COM");
+
+    expect(result).toEqual({ authoritative: true, reason: "domain_verified" });
+  });
+
+  it("matches subdomains of verified domain", async () => {
+    setupMocks({ verifiedDomains: ["acme.com"] });
+    service = new SamlAccountLinkingService(mockPrismaClient);
+
+    const result = await service.isSamlIdpAuthoritativeForEmail(ORG_TEAM_ID, "user@sales.acme.com");
+
+    expect(result).toEqual({ authoritative: true, reason: "domain_verified" });
+  });
+
+  it("returns not authoritative when domain doesn't match and user is not member", async () => {
+    setupMocks({ verifiedDomains: ["acme.com"], hasMembership: false });
+    service = new SamlAccountLinkingService(mockPrismaClient);
+
+    const result = await service.isSamlIdpAuthoritativeForEmail(ORG_TEAM_ID, "user@different.com");
+
+    expect(result).toEqual({ authoritative: false, reason: "domain_mismatch" });
+  });
+
+  it("returns authoritative for existing org member with different domain", async () => {
+    setupMocks({ verifiedDomains: ["acme.com"], hasMembership: true });
+    service = new SamlAccountLinkingService(mockPrismaClient);
+
+    const result = await service.isSamlIdpAuthoritativeForEmail(ORG_TEAM_ID, "user@personal-email.com");
+
+    expect(result).toEqual({ authoritative: true, reason: "existing_member" });
+  });
+
+  it("skips membership check when domain matches", async () => {
+    setupMocks({ verifiedDomains: ["acme.com"], hasMembership: true });
+    service = new SamlAccountLinkingService(mockPrismaClient);
+
+    await service.isSamlIdpAuthoritativeForEmail(ORG_TEAM_ID, "user@acme.com");
+
+    expect(hasAcceptedMembershipSpy).not.toHaveBeenCalled();
+  });
+
+  it("returns not authoritative for invalid email", async () => {
+    setupMocks({});
+    service = new SamlAccountLinkingService(mockPrismaClient);
+
+    const result = await service.isSamlIdpAuthoritativeForEmail(ORG_TEAM_ID, "invalid-email");
+
+    expect(result).toEqual({ authoritative: false, reason: "invalid_email" });
+    expect(getVerifiedDomainsSpy).not.toHaveBeenCalled();
+  });
+
+  it("returns not authoritative for empty email", async () => {
+    setupMocks({});
+    service = new SamlAccountLinkingService(mockPrismaClient);
+
+    const result = await service.isSamlIdpAuthoritativeForEmail(ORG_TEAM_ID, "");
+
+    expect(result).toEqual({ authoritative: false, reason: "invalid_email" });
+  });
+
+  it("falls back to membership when org has no verified domains", async () => {
+    setupMocks({ verifiedDomains: [], hasMembership: true });
+    service = new SamlAccountLinkingService(mockPrismaClient);
+
+    const result = await service.isSamlIdpAuthoritativeForEmail(ORG_TEAM_ID, "user@any-domain.com");
+
+    expect(result).toEqual({ authoritative: true, reason: "existing_member" });
+  });
+
+  it("rejects when org has no verified domains and user is not member", async () => {
+    setupMocks({ verifiedDomains: [], hasMembership: false });
+    service = new SamlAccountLinkingService(mockPrismaClient);
+
+    const result = await service.isSamlIdpAuthoritativeForEmail(ORG_TEAM_ID, "attacker@evil.com");
+
+    expect(result).toEqual({ authoritative: false, reason: "domain_mismatch" });
+  });
+});
+
+describe("validateSamlAccountConversion", () => {
+  beforeEach(() => {
+    vi.restoreAllMocks();
+    setupMocks({ verifiedDomains: [], hasMembership: false });
+  });
+
+  it("blocks when no SAML tenant provided (deny by default)", async () => {
+    const result = await validateSamlAccountConversion(undefined, "user@example.com", "CAL→SAML");
+    expect(result).toEqual({
+      allowed: false,
+      errorUrl: "/auth/error?error=saml-idp-not-authoritative",
+    });
+  });
+
+  it("allows when tenant is not org-based", async () => {
+    const result = await validateSamlAccountConversion("Cal.com", "user@example.com", "CAL→SAML");
+    expect(result).toEqual({ allowed: true });
+  });
+
+  it("blocks when IdP is not authoritative", async () => {
+    const result = await validateSamlAccountConversion("team-123", "attacker@evil.com", "CAL→SAML");
+
+    expect(result).toEqual({
+      allowed: false,
+      errorUrl: "/auth/error?error=saml-idp-not-authoritative",
+    });
+  });
+
+  it("allows when IdP is authoritative", async () => {
+    setupMocks({ verifiedDomains: ["acme.com"] });
+
+    const result = await validateSamlAccountConversion("team-123", "user@acme.com", "CAL→SAML");
+
+    expect(result).toEqual({ allowed: true });
+  });
+});
+
+describe("Security: SAML Account Takeover Prevention", () => {
+  beforeEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("blocks takeover when attacker org asserts victim's email", async () => {
+    setupMocks({ verifiedDomains: ["attacker-org.com"], hasMembership: false });
+
+    const result = await validateSamlAccountConversion("team-999", "victim@gmail.com", "Google→SAML");
+
+    expect(result.allowed).toBe(false);
+  });
+
+  it("allows SSO when org owns the email domain", async () => {
+    setupMocks({ verifiedDomains: ["acme.com"] });
+
+    const result = await validateSamlAccountConversion("team-100", "employee@acme.com", "CAL→SAML");
+
+    expect(result).toEqual({ allowed: true });
+  });
+
+  it("allows SSO for existing members with personal email", async () => {
+    setupMocks({ verifiedDomains: ["acme.com"], hasMembership: true });
+
+    const service = new SamlAccountLinkingService(mockPrismaClient);
+    const result = await service.isSamlIdpAuthoritativeForEmail(100, "contractor@personal-email.com");
+
+    expect(result.authoritative).toBe(true);
+    expect(result.reason).toBe("existing_member");
+  });
+
+  it("blocks invite takeover when attacker org claims invited user", async () => {
+    // Org A invites victim@gmail.com, attacker in Org B tries to claim via SAML
+    setupMocks({ verifiedDomains: ["attacker-org.com"], hasMembership: false });
+
+    const result = await validateSamlAccountConversion("team-999", "victim@gmail.com", "Invite→SAML");
+
+    expect(result.allowed).toBe(false);
+  });
+});
diff --git a/packages/features/auth/lib/samlAccountLinking.ts b/packages/features/auth/lib/samlAccountLinking.ts
new file mode 100644
index 0000000000..5a5f3d45a8
--- /dev/null
+++ b/packages/features/auth/lib/samlAccountLinking.ts
@@ -0,0 +1,96 @@
+import { MembershipRepository } from "@calcom/features/membership/repositories/MembershipRepository";
+import { OrganizationSettingsRepository } from "@calcom/features/organizations/repositories/OrganizationSettingsRepository";
+import logger from "@calcom/lib/logger";
+import { prisma } from "@calcom/prisma";
+import type { PrismaClient } from "@calcom/prisma";
+
+import { tenantPrefix } from "../../ee/sso/lib/saml";
+
+const log = logger.getSubLogger({ prefix: ["samlAccountLinking"] });
+const SAML_NOT_AUTHORITATIVE_ERROR_URL = "/auth/error?error=saml-idp-not-authoritative";
+
+export function getTeamIdFromSamlTenant(tenant: string): number | null {
+  if (!tenant.startsWith(tenantPrefix)) {
+    return null;
+  }
+  const teamId = parseInt(tenant.replace(tenantPrefix, ""), 10);
+  return isNaN(teamId) ? null : teamId;
+}
+
+/**
+ * Prevents account takeover via malicious SAML IdPs asserting arbitrary emails.
+ * IdP is authoritative when domain matches org's verified domain or user is already a member.
+ */
+export class SamlAccountLinkingService {
+  private membershipRepository: MembershipRepository;
+  private orgSettingsRepository: OrganizationSettingsRepository;
+
+  constructor(prismaClient: PrismaClient = prisma) {
+    this.membershipRepository = new MembershipRepository(prismaClient);
+    this.orgSettingsRepository = new OrganizationSettingsRepository(prismaClient);
+  }
+
+  async isSamlIdpAuthoritativeForEmail(
+    samlOrgTeamId: number,
+    email: string
+  ): Promise<{ authoritative: boolean; reason: string }> {
+    const emailDomain = email.split("@")[1]?.toLowerCase();
+
+    if (!emailDomain) {
+      return { authoritative: false, reason: "invalid_email" };
+    }
+
+    const verifiedDomains = await this.orgSettingsRepository.getVerifiedDomains(samlOrgTeamId);
+    const domainMatches = verifiedDomains.some(
+      (verified) => emailDomain === verified || emailDomain.endsWith(`.${verified}`)
+    );
+    if (domainMatches) {
+      return { authoritative: true, reason: "domain_verified" };
+    }
+
+    const hasMembership = await this.membershipRepository.hasAcceptedMembershipByEmail({
+      email,
+      teamId: samlOrgTeamId,
+    });
+
+    if (hasMembership) {
+      return { authoritative: true, reason: "existing_member" };
+    }
+
+    return { authoritative: false, reason: "domain_mismatch" };
+  }
+}
+
+export type AccountConversionValidationResult =
+  | { allowed: true }
+  | { allowed: false; errorUrl: string };
+
+export async function validateSamlAccountConversion(
+  samlTenant: string | undefined,
+  email: string,
+  conversionContext: string
+): Promise<AccountConversionValidationResult> {
+  if (!samlTenant) {
+    // Deny by default - if tenant is missing, we cannot verify IdP authority
+    log.error("SAML conversion blocked - missing tenant", { emailDomain: email.split("@")[1], conversionContext });
+    return { allowed: false, errorUrl: SAML_NOT_AUTHORITATIVE_ERROR_URL };
+  }
+
+  const samlOrgTeamId = getTeamIdFromSamlTenant(samlTenant);
+  if (!samlOrgTeamId) {
+    return { allowed: true };
+  }
+
+  const service = new SamlAccountLinkingService(prisma);
+  const authority = await service.isSamlIdpAuthoritativeForEmail(samlOrgTeamId, email);
+
+  if (!authority.authoritative) {
+    log.warn(`Blocking ${conversionContext} conversion - IdP not authoritative`, {
+      emailDomain: email.split("@")[1],
+      samlOrgTeamId,
+      reason: authority.reason,
+    });
+    return { allowed: false, errorUrl: SAML_NOT_AUTHORITATIVE_ERROR_URL };
+  }
+  return { allowed: true };
+}
diff --git a/packages/features/membership/repositories/MembershipRepository.ts b/packages/features/membership/repositories/MembershipRepository.ts
index 4004ba8f46..0fea70bd5f 100644
--- a/packages/features/membership/repositories/MembershipRepository.ts
+++ b/packages/features/membership/repositories/MembershipRepository.ts
@@ -560,4 +560,29 @@ export class MembershipRepository {
       },
     });
   }
+
+  // Two indexed lookups instead of JOIN with ILIKE (which bypasses index)
+  async hasAcceptedMembershipByEmail({
+    email,
+    teamId,
+  }: {
+    email: string;
+    teamId: number;
+  }): Promise<boolean> {
+    const user = await this.prismaClient.user.findUnique({
+      where: { email: email.toLowerCase() },
+      select: { id: true },
+    });
+
+    if (!user) return false;
+
+    const membership = await this.prismaClient.membership.findUnique({
+      where: {
+        userId_teamId: { userId: user.id, teamId },
+      },
+      select: { accepted: true },
+    });
+
+    return membership?.accepted ?? false;
+  }
 }
diff --git a/packages/features/organizations/repositories/OrganizationSettingsRepository.ts b/packages/features/organizations/repositories/OrganizationSettingsRepository.ts
index 9e31aa20b7..c99375d24d 100644
--- a/packages/features/organizations/repositories/OrganizationSettingsRepository.ts
+++ b/packages/features/organizations/repositories/OrganizationSettingsRepository.ts
@@ -1,7 +1,7 @@
 import type { PrismaClient } from "@calcom/prisma";
 
 export class OrganizationSettingsRepository {
-  constructor(private prismaClient: PrismaClient) {}
+  constructor(private readonly prismaClient: PrismaClient) {}
 
   async getEmailSettings(organizationId: number) {
     return await this.prismaClient.organizationSettings.findUnique({
@@ -19,4 +19,22 @@ export class OrganizationSettingsRepository {
       },
     });
   }
+
+  // Returns array for future multi-domain support
+  async getVerifiedDomains(organizationId: number): Promise<string[]> {
+    const settings = await this.prismaClient.organizationSettings.findUnique({
+      where: { organizationId },
+      select: {
+        isOrganizationVerified: true,
+        orgAutoAcceptEmail: true,
+      },
+    });
+
+    if (!settings?.isOrganizationVerified) {
+      return [];
+    }
+
+    const domain = settings.orgAutoAcceptEmail;
+    return domain ? [domain.toLowerCase()] : [];
+  }
 }