diff --git a/src/CredentialsClient.ts b/src/CredentialsClient.ts
index ef8beed..43c16b1 100644
--- a/src/CredentialsClient.ts
+++ b/src/CredentialsClient.ts
@@ -4,7 +4,7 @@ import { defaultProvider } from '@aws-sdk/credential-provider-node';
 import type { AwsCredentialIdentity } from '@aws-sdk/types';
 import { NodeHttpHandler } from '@smithy/node-http-handler';
 import { ProxyAgent } from 'proxy-agent';
-import { buildCustomUserAgent, errorMessage, getCallerIdentity } from './helpers';
+import { buildCustomUserAgent, errorMessage, getCallerIdentity, maskProxyCredentials } from './helpers';
 import { ProxyResolver } from './ProxyResolver';
 
 if (!process.env.AWS_EXECUTION_ENV) {
@@ -32,6 +32,7 @@ export class CredentialsClient {
     }
     if (props.proxyServer) {
       info('Configuring proxy handler for STS client');
+      maskProxyCredentials(props.proxyServer);
       const proxyOptions: { httpProxy: string; httpsProxy: string; noProxy?: string } = {
         httpProxy: props.proxyServer,
         httpsProxy: props.proxyServer,
diff --git a/src/helpers.ts b/src/helpers.ts
index 27524ec..c65f297 100644
--- a/src/helpers.ts
+++ b/src/helpers.ts
@@ -193,6 +193,29 @@ export function toCredentialIdentity(creds?: Partial<Credentials>): AwsCredentia
   };
 }
 
+// Registers any userinfo embedded in a proxy URL as secrets so it is masked in job logs.
+// First the literal proxy string, then any username/password components if parseable.
+// If the username/password is percent-encoded, the decoded form is also masked.
+export function maskProxyCredentials(proxyServer: string): void {
+  core.setSecret(proxyServer);
+  let url: URL;
+  try {
+    url = new URL(proxyServer);
+  } catch (_) {
+    return;
+  }
+  for (const part of [url.username, url.password]) {
+    if (!part) continue;
+    core.setSecret(part);
+    try {
+      const decoded = decodeURIComponent(part);
+      if (decoded !== part) core.setSecret(decoded);
+    } catch (_) {
+      // malformed percent-encoding; the raw form is already masked
+    }
+  }
+}
+
 // Tags have a more restrictive set of acceptable characters than GitHub environment variables can.
 // This replaces anything not conforming to the tag restrictions by inverting the regular expression.
 // See the AWS documentation for constraint specifics https://docs.aws.amazon.com/STS/latest/APIReference/API_Tag.html.
diff --git a/test/helpers.test.ts b/test/helpers.test.ts
index eef1eb9..71a7c8e 100644
--- a/test/helpers.test.ts
+++ b/test/helpers.test.ts
@@ -126,6 +126,29 @@ describe('Configure AWS Credentials helpers', {}, () => {
     expect(core.exportVariable).toHaveBeenCalledWith('AWS_SESSION_TOKEN', '');
   });
 
+  describe('maskProxyCredentials', {}, () => {
+    it('masks username and password embedded in a proxy URL', {}, () => {
+      helpers.maskProxyCredentials('http://user:secretpass@proxy.example.com:8080');
+      expect(core.setSecret).toHaveBeenCalledWith('user');
+      expect(core.setSecret).toHaveBeenCalledWith('secretpass');
+    });
+
+    it('masks both encoded and decoded forms of the credentials', {}, () => {
+      helpers.maskProxyCredentials('http://user:p%40ss@proxy.example.com:8080');
+      expect(core.setSecret).toHaveBeenCalledWith('p%40ss');
+      expect(core.setSecret).toHaveBeenCalledWith('p@ss');
+    });
+
+    it('masks the whole value even without embedded credentials or when unparseable', {}, () => {
+      helpers.maskProxyCredentials('http://proxy.example.com:8080');
+      expect(core.setSecret).toHaveBeenCalledWith('http://proxy.example.com:8080');
+      helpers.maskProxyCredentials('not a url');
+      expect(core.setSecret).toHaveBeenCalledWith('not a url');
+      // no username/password parts, so exactly one mask per call
+      expect(core.setSecret).toHaveBeenCalledTimes(2);
+    });
+  });
+
   describe('validateAccountId', {}, () => {
     it('enforces the allow-list even when the first element is empty', {}, () => {
       expect(() => helpers.validateAccountId(['', '999999999999'], '111111111111')).toThrow(/does not match/);
diff --git a/test/index.test.ts b/test/index.test.ts
index bc7d4f4..7b9aafd 100644
--- a/test/index.test.ts
+++ b/test/index.test.ts
@@ -1267,6 +1267,21 @@ describe('Configure AWS Credentials', {}, () => {
 
       expect(core.setFailed).not.toHaveBeenCalled();
     });
+
+    it('masks credentials embedded in the proxy URL', async () => {
+      vi.mocked(core.getInput).mockImplementation(
+        mocks.getInput({
+          ...mocks.GH_OIDC_INPUTS,
+          'http-proxy': 'http://user:secretpass@proxy.example.com:8080',
+        }),
+      );
+
+      await run();
+
+      expect(core.setSecret).toHaveBeenCalledWith('user');
+      expect(core.setSecret).toHaveBeenCalledWith('secretpass');
+      expect(core.setFailed).not.toHaveBeenCalled();
+    });
   });
 
   describe('AWS Profile Support', {}, () => {