diff --git a/frappe/tests/test_oauth20.py b/frappe/tests/test_oauth20.py
index 8375d42851..533f2a49a7 100644
--- a/frappe/tests/test_oauth20.py
+++ b/frappe/tests/test_oauth20.py
@@ -12,6 +12,7 @@ from frappe.integrations.oauth2 import encode_params
 from frappe.tests import IntegrationTestCase
 from frappe.tests.test_api import get_test_client, make_request, suppress_stdout
 from frappe.tests.utils import make_test_records
+from frappe.utils.oauth import build_oauth_url
 
 if TYPE_CHECKING:
 	from frappe.integrations.doctype.social_login_key.social_login_key import SocialLoginKey
@@ -360,6 +361,28 @@ class TestOAuth20(FrappeRequestTestCase):
 
 		self.assertTrue(payload.get("nonce") == nonce)
 
+	def test_build_oauth_url(self):
+		self.assertEqual(build_oauth_url("https://example.com", "/endpoint"), "https://example.com/endpoint")
+
+		self.assertEqual(build_oauth_url("https://example.com"), "https://example.com")
+
+		self.assertEqual(build_oauth_url("https://example.com", None), "https://example.com")
+
+		self.assertEqual(
+			build_oauth_url("https://example.com", "//endpoint.com/test"),
+			"https://example.com//endpoint.com/test",
+		)
+
+		self.assertEqual(
+			build_oauth_url("https://example.com", "http://endpoint.com/test"), "http://endpoint.com/test"
+		)
+
+		self.assertEqual(
+			build_oauth_url("https://example.com", "https://endpoint.com"), "https://endpoint.com"
+		)
+
+		self.assertEqual(build_oauth_url("https://example.com", ""), "https://example.com")
+
 	def decode_id_token(self, id_token):
 		import jwt
 
diff --git a/frappe/utils/oauth.py b/frappe/utils/oauth.py
index d41304084b..8f7c7a6d1f 100644
--- a/frappe/utils/oauth.py
+++ b/frappe/utils/oauth.py
@@ -5,6 +5,7 @@ import base64
 import json
 from collections.abc import Callable
 from typing import TYPE_CHECKING
+from urllib.parse import urlparse
 
 import frappe
 import frappe.utils
@@ -19,18 +20,50 @@ if TYPE_CHECKING:
 class SignupDisabledError(frappe.PermissionError): ...
 
 
+def build_oauth_url(base_url: str, url: str | None = None) -> str:
+	"""
+	Build a complete OAuth authorization URL.
+
+	This helper constructs a full OAuth URL starting from a given base URL.
+
+	If `url` is omitted, the function simply returns the normalized base URL.  If the
+	`url` contains the relative or absolute path, the function will return this
+	appended to the base URL.  If the `url` contains a `scheme` (e.g. "https://" and a
+	`netloc` (e.g. "www.example.com")), the function will return the passed `url` alone.
+
+	Args:
+		base_url (str): The base OAuth endpoint (e.g. "https://example.com").
+		url (str | None): An optional path or override URL to combine with the base.
+
+	Returns:
+		str: The fully qualified OAuth URL ready for use in redirects or API calls.
+	"""
+	if url is None:
+		return base_url
+	parsed = urlparse(url)
+	if not (parsed.scheme and parsed.netloc):
+		return base_url + url
+	return url
+
+
 def get_oauth2_providers() -> dict[str, dict]:
 	out = {}
 	providers = frappe.get_all("Social Login Key", fields=["*"])
 	for provider in providers:
-		authorize_url, access_token_url = provider.authorize_url, provider.access_token_url
+		authorize_url, access_token_url, api_endpoint_url = (
+			provider.authorize_url,
+			provider.access_token_url,
+			provider.api_endpoint,
+		)
+
 		if provider.custom_base_url:
-			authorize_url = provider.base_url + provider.authorize_url
-			access_token_url = provider.base_url + provider.access_token_url
+			authorize_url = build_oauth_url(provider.base_url, provider.authorize_url)
+			access_token_url = build_oauth_url(provider.base_url, provider.access_token_url)
+			api_endpoint_url = build_oauth_url(provider.base_url, provider.api_endpoint)
 
 		# Keycloak needs this, the base URL also has a route, that urljoin() ignores
 		if provider.name == "keycloak":
-			provider.api_endpoint = provider.base_url + provider.api_endpoint
+			api_endpoint_url = build_oauth_url(provider.base_url, provider.api_endpoint)
 
 		out[provider.name] = {
 			"flow_params": {
@@ -40,7 +73,7 @@ def get_oauth2_providers() -> dict[str, dict]:
 				"base_url": provider.base_url,
 			},
 			"redirect_uri": provider.redirect_url,
-			"api_endpoint": provider.api_endpoint,
+			"api_endpoint": api_endpoint_url,
 		}
 		if provider.auth_url_data:
 			out[provider.name]["auth_url_data"] = json.loads(provider.auth_url_data)