diff --git a/packages/profile-sync-controller/CHANGELOG.md b/packages/profile-sync-controller/CHANGELOG.md
index e371bbd9083..d66c90c8213 100644
--- a/packages/profile-sync-controller/CHANGELOG.md
+++ b/packages/profile-sync-controller/CHANGELOG.md
@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Coalesce overlapping `performSignIn` calls so only one sign-in runs at a time ([#10646](https://github.com/MetaMask/core/pull/10646))
 - Rely on `@metamask/key-tree` crypto implementation for HMAC-SHA-512 instead of hardcoded `noble` implementation ([#10424](https://github.com/MetaMask/core/pull/10424))
   - `@metamask/key-tree` uses WebCrypto API if available and fallback to `noble` otherwise.
   - One note, we expect the platform to provide a fully-compliant WebCrypto (`crypto.subtle`) implementation for this to work (`@metamask/key-tree` detection is global and not "per crypto functions").
diff --git a/packages/profile-sync-controller/src/controllers/authentication/AuthenticationController.test.ts b/packages/profile-sync-controller/src/controllers/authentication/AuthenticationController.test.ts
index cee2884326d..8632da513a1 100644
--- a/packages/profile-sync-controller/src/controllers/authentication/AuthenticationController.test.ts
+++ b/packages/profile-sync-controller/src/controllers/authentication/AuthenticationController.test.ts
@@ -596,6 +596,26 @@ describe('AuthenticationController', () => {
       expect(controller.state.needsProfilePairing).toBe(false);
     });
 
+    it('shares one in-flight performSignIn across overlapping callers', async () => {
+      const metametrics = createMockAuthMetaMetrics();
+      arrangeAuthAPIs({ mockPairProfilesDelayMs: 50 });
+      const { messenger } = createMockAuthenticationMessenger();
+
+      const controller = new AuthenticationController({
+        messenger,
+        metametrics,
+      });
+
+      const first = controller.performSignIn();
+      const second = controller.performSignIn();
+      const [firstTokens, secondTokens] = await Promise.all([first, second]);
+
+      expect(secondTokens).toBe(firstTokens);
+
+      const thirdTokens = await controller.performSignIn();
+      expect(thirdTokens).not.toBe(firstTokens);
+    });
+
     it('does NOT throw out of performSignIn when pairProfiles fails, leaves needsProfilePairing=true', async () => {
       const metametrics = createMockAuthMetaMetrics();
       arrangeAuthAPIs({
diff --git a/packages/profile-sync-controller/src/controllers/authentication/AuthenticationController.ts b/packages/profile-sync-controller/src/controllers/authentication/AuthenticationController.ts
index d73cb4cdb4e..bce495dfd0e 100644
--- a/packages/profile-sync-controller/src/controllers/authentication/AuthenticationController.ts
+++ b/packages/profile-sync-controller/src/controllers/authentication/AuthenticationController.ts
@@ -333,6 +333,11 @@ export class AuthenticationController extends BaseController<
    */
   #profilePairingRequestEpoch = 0;
 
+  /**
+   * Sign-in currently running. Overlapping callers share it.
+   */
+  #ongoingSignIn?: Promise<string[]>;
+
   readonly #keyringController = {
     setupLockedStateSubscriptions: () => {
       const { isUnlocked } = this.messenger.call('KeyringController:getState');
@@ -596,7 +601,21 @@ export class AuthenticationController extends BaseController<
 
   public async performSignIn(): Promise<string[]> {
     this.#assertIsUnlocked('performSignIn');
+    if (this.#ongoingSignIn) {
+      return await this.#ongoingSignIn;
+    }
+
+    const signIn = this.#runPerformSignIn().finally(() => {
+      if (this.#ongoingSignIn === signIn) {
+        this.#ongoingSignIn = undefined;
+      }
+    });
+
+    this.#ongoingSignIn = signIn;
+    return await signIn;
+  }
 
+  async #runPerformSignIn(): Promise<string[]> {
     const epochAtStart = this.#profilePairingRequestEpoch;
     const entropySourceIds = this.#getHdKeyringEntropySourceIds();
     const accessTokens: string[] = [];