diff --git a/authentik/admin/files/backends/s3.py b/authentik/admin/files/backends/s3.py
index b6f601f18b..099597afa3 100644
--- a/authentik/admin/files/backends/s3.py
+++ b/authentik/admin/files/backends/s3.py
@@ -173,7 +173,22 @@ class S3Backend(ManageableBackend):
             if custom_domain:
                 parsed = urlsplit(url)
                 scheme = "https" if use_https else "http"
-                url = f"{scheme}://{custom_domain}{parsed.path}?{parsed.query}"
+                path = parsed.path
+
+                # When using path-style addressing, the presigned URL contains the bucket
+                # name in the path (e.g., /bucket-name/key). Since custom_domain must
+                # include the bucket name (per docs), strip it from the path to avoid
+                # duplication. See: https://github.com/goauthentik/authentik/issues/19521
+                # Check with trailing slash to ensure exact bucket name match
+                if path.startswith(f"/{self.bucket_name}/"):
+                    path = path.removeprefix(f"/{self.bucket_name}")
+
+                # Normalize to avoid double slashes
+                custom_domain = custom_domain.rstrip("/")
+                if not path.startswith("/"):
+                    path = f"/{path}"
+
+                url = f"{scheme}://{custom_domain}{path}?{parsed.query}"
 
             return url
 
diff --git a/authentik/admin/files/backends/tests/test_s3_backend.py b/authentik/admin/files/backends/tests/test_s3_backend.py
index 2c5330efae..b8fee39640 100644
--- a/authentik/admin/files/backends/tests/test_s3_backend.py
+++ b/authentik/admin/files/backends/tests/test_s3_backend.py
@@ -110,3 +110,38 @@ class TestS3Backend(FileTestS3BackendMixin, TestCase):
         """Test S3Backend with REPORTS usage"""
         self.assertEqual(self.reports_s3_backend.usage, FileUsage.REPORTS)
         self.assertEqual(self.reports_s3_backend.base_path, "reports/public")
+
+    @CONFIG.patch("storage.s3.secure_urls", True)
+    @CONFIG.patch("storage.s3.addressing_style", "path")
+    def test_file_url_custom_domain_with_bucket_no_duplicate(self):
+        """Test file_url doesn't duplicate bucket name when custom_domain includes bucket.
+
+        Regression test for https://github.com/goauthentik/authentik/issues/19521
+
+        When using:
+        - Path-style addressing (bucket name goes in URL path, not subdomain)
+        - Custom domain that includes the bucket name (e.g., s3.example.com/bucket-name)
+
+        The bucket name should NOT appear twice in the final URL.
+
+        Example of the bug:
+        - custom_domain = "s3.example.com/authentik-media"
+        - boto3 presigned URL = "http://s3.example.com/authentik-media/media/public/file.png?..."
+        - Buggy result = "https://s3.example.com/authentik-media/authentik-media/media/public/file.png?..."
+        """
+        bucket_name = self.media_s3_bucket_name
+
+        # Custom domain includes the bucket name
+        custom_domain = f"localhost:8020/{bucket_name}"
+
+        with CONFIG.patch("storage.media.s3.custom_domain", custom_domain):
+            url = self.media_s3_backend.file_url("application-icons/test.svg", use_cache=False)
+
+        # The bucket name should appear exactly once in the URL path, not twice
+        bucket_occurrences = url.count(bucket_name)
+        self.assertEqual(
+            bucket_occurrences,
+            1,
+            f"Bucket name '{bucket_name}' appears {bucket_occurrences} times in URL, expected 1. "
+            f"URL: {url}",
+        )