diff --git a/.changeset/slow-drinks-enjoy.md b/.changeset/slow-drinks-enjoy.md
new file mode 100644
index 0000000000..5a171b8fd1
--- /dev/null
+++ b/.changeset/slow-drinks-enjoy.md
@@ -0,0 +1,5 @@
+---
+'@backstage/integration-aws-node': patch
+---
+
+Fixed bug in DefaultAwsCredentialsManager where aws.mainAccount.region has no effect on the STS region used for account ID lookup during credential provider lookup when falling back to the main account, and it does not default to us-east-1
diff --git a/packages/integration-aws-node/src/DefaultAwsCredentialsManager.test.ts b/packages/integration-aws-node/src/DefaultAwsCredentialsManager.test.ts
index d81f717a58..582b003724 100644
--- a/packages/integration-aws-node/src/DefaultAwsCredentialsManager.test.ts
+++ b/packages/integration-aws-node/src/DefaultAwsCredentialsManager.test.ts
@@ -91,6 +91,18 @@ describe('DefaultAwsCredentialsManager', () => {
       Account: '123456789012',
     });
 
+    stsMock
+      .on(GetCallerIdentityCommand)
+      .callsFake(async (_input, getClient) => {
+        const client = getClient();
+        const region = await client.config.region();
+        if (!region) {
+          throw new Error('Region is missing');
+        }
+        return {
+          Account: '123456789012',
+        };
+      });
     stsMock
       .on(AssumeRoleCommand, {
         RoleArn: 'arn:aws:iam::111111111111:role/hello',
@@ -488,5 +500,21 @@ describe('DefaultAwsCredentialsManager', () => {
         },
       });
     });
+
+    it('passes mainAccount region to fillInAccountId for account ID lookup during fallback', async () => {
+      const region = 'us-west-2';
+      const configWithRegion = new ConfigReader({
+        aws: {
+          mainAccount: {
+            region,
+          },
+        },
+      });
+      const provider =
+        DefaultAwsCredentialsManager.fromConfig(configWithRegion);
+      await provider.getCredentialProvider({ accountId: '123456789012' });
+
+      expect(await stsMock.call(0).thisValue.config.region()).toEqual(region);
+    });
   });
 });
diff --git a/packages/integration-aws-node/src/DefaultAwsCredentialsManager.ts b/packages/integration-aws-node/src/DefaultAwsCredentialsManager.ts
index 3048fdfe40..b54a4fda26 100644
--- a/packages/integration-aws-node/src/DefaultAwsCredentialsManager.ts
+++ b/packages/integration-aws-node/src/DefaultAwsCredentialsManager.ts
@@ -37,6 +37,7 @@ import { Config } from '@backstage/config';
 
 /**
  * Retrieves the account ID for the given credential provider from STS.
+ * Include the region if present, otherwise use the default region.
  */
 async function fillInAccountId(credProvider: AwsCredentialProvider) {
   if (credProvider.accountId) {
@@ -44,7 +45,7 @@ async function fillInAccountId(credProvider: AwsCredentialProvider) {
   }
 
   const client = new STSClient({
-    region: credProvider.stsRegion,
+    region: credProvider.stsRegion ?? 'us-east-1',
     customUserAgent: 'backstage-aws-credentials-manager',
     credentialDefaultProvider: () => credProvider.sdkCredentialProvider,
   });
@@ -174,6 +175,7 @@ export class DefaultAwsCredentialsManager implements AwsCredentialsManager {
       awsConfig.mainAccount,
     );
     const mainAccountCredProvider: AwsCredentialProvider = {
+      stsRegion: awsConfig.mainAccount.region,
       sdkCredentialProvider: mainAccountSdkCredProvider,
     };