diff --git a/packages/javascript/src/api/__tests__/executeEmbeddedSignInFlow.test.ts b/packages/javascript/src/api/__tests__/executeEmbeddedSignInFlow.test.ts
index bca7908..25d1947 100644
--- a/packages/javascript/src/api/__tests__/executeEmbeddedSignInFlow.test.ts
+++ b/packages/javascript/src/api/__tests__/executeEmbeddedSignInFlow.test.ts
@@ -34,6 +34,12 @@ const captureRequestBody = (): Record<string, unknown> => {
   return JSON.parse(requestInit.body as string) as Record<string, unknown>;
 };
 
+const captureRequestHeaders = (): Record<string, string> => {
+  const calls = (fetch as ReturnType<typeof vi.fn>).mock.calls;
+  const requestInit = calls[calls.length - 1][1] as RequestInit;
+  return requestInit.headers as Record<string, string>;
+};
+
 describe('executeEmbeddedSignInFlow', (): void => {
   beforeEach((): void => {
     vi.resetAllMocks();
@@ -134,6 +140,37 @@ describe('executeEmbeddedSignInFlow', (): void => {
     });
   });
 
+  describe('Flow-Secret header', (): void => {
+    it('sends the Flow Secret header on a new flow start', async (): Promise<void> => {
+      await executeEmbeddedSignInFlow({
+        flowSecret: 'flow-secret-123',
+        payload: {applicationId: 'app-1', flowType: 'AUTHENTICATION'},
+        url: URL,
+      });
+
+      expect(captureRequestHeaders()).toMatchObject({'Flow-Secret': 'flow-secret-123'});
+    });
+
+    it('does NOT send the Flow Secret header on a step submission', async (): Promise<void> => {
+      await executeEmbeddedSignInFlow({
+        flowSecret: 'flow-secret-123',
+        payload: {action: 'submit', executionId: 'exec-abc', inputs: {password: 'secret', username: 'user'}},
+        url: URL,
+      });
+
+      expect(captureRequestHeaders()).not.toHaveProperty('Flow-Secret');
+    });
+
+    it('does NOT send the Flow Secret header when no flowSecret is provided', async (): Promise<void> => {
+      await executeEmbeddedSignInFlow({
+        payload: {applicationId: 'app-1', flowType: 'AUTHENTICATION'},
+        url: URL,
+      });
+
+      expect(captureRequestHeaders()).not.toHaveProperty('Flow-Secret');
+    });
+  });
+
   it('throws when payload is missing', async (): Promise<void> => {
     await expect(executeEmbeddedSignInFlow({url: URL})).rejects.toThrow('Authorization payload is required');
   });
diff --git a/packages/javascript/src/api/executeEmbeddedSignInFlow.ts b/packages/javascript/src/api/executeEmbeddedSignInFlow.ts
index 156c8b2..6eeb80e 100644
--- a/packages/javascript/src/api/executeEmbeddedSignInFlow.ts
+++ b/packages/javascript/src/api/executeEmbeddedSignInFlow.ts
@@ -26,6 +26,7 @@ const executeEmbeddedSignInFlow = async ({
   baseUrl,
   payload,
   authId,
+  flowSecret,
   ...requestConfig
 }: EmbeddedFlowExecuteRequestConfig): Promise<EmbeddedSignInFlowResponse> => {
   if (!payload) {
@@ -74,6 +75,9 @@ const executeEmbeddedSignInFlow = async ({
     headers: {
       Accept: 'application/json',
       'Content-Type': 'application/json',
+      // A backend/server-side application presents its Flow Secret to initiate a new flow. It is
+      // only relevant on initiation, so it is omitted from continuation requests.
+      ...(isNewFlowStart && flowSecret ? {'Flow-Secret': flowSecret} : {}),
       ...requestConfig.headers,
     } as HeadersInit,
     method: requestConfig.method || 'POST',
diff --git a/packages/javascript/src/models/embedded-flow.ts b/packages/javascript/src/models/embedded-flow.ts
index 780194b..234b3da 100644
--- a/packages/javascript/src/models/embedded-flow.ts
+++ b/packages/javascript/src/models/embedded-flow.ts
@@ -35,6 +35,13 @@ export enum EmbeddedFlowResponseType {
  */
 export interface EmbeddedFlowExecuteRequestConfigBase<T = any> extends Partial<Request> {
   baseUrl?: string;
+  /**
+   * Flow Secret authenticating a backend/server-side application when it initiates a new native
+   * flow directly. When set, it is sent in the `Flow-Secret` request header, and only on a new
+   * flow initiation (a payload carrying `applicationId` and `flowType`) — never on continuation
+   * requests. Not applicable to public clients or redirect-based applications.
+   */
+  flowSecret?: string;
   payload?: T;
   url?: string;
 }
diff --git a/packages/nextjs/src/ThunderIDNextClient.ts b/packages/nextjs/src/ThunderIDNextClient.ts
index 0963fae..50cfa0f 100644
--- a/packages/nextjs/src/ThunderIDNextClient.ts
+++ b/packages/nextjs/src/ThunderIDNextClient.ts
@@ -265,6 +265,7 @@ class ThunderIDNextClient<T extends ThunderIDNextConfig = ThunderIDNextConfig> e
 
       return executeEmbeddedSignInFlow({
         baseUrl: configData?.baseUrl,
+        flowSecret: arg2?.flowSecret,
         payload: arg1,
         url: arg2?.url,
       }) as unknown as Promise<EmbeddedSignInFlowResponse>;