diff --git a/requests/sessions.py b/requests/sessions.py
index 6cb3b4dae3..dbcf2a7b0e 100644
--- a/requests/sessions.py
+++ b/requests/sessions.py
@@ -324,7 +324,9 @@ def rebuild_proxies(self, prepared_request, proxies):
         except KeyError:
             username, password = None, None
 
-        if username and password:
+        # urllib3 handles proxy authorization for us in the standard adapter.
+        # Avoid appending this to TLS tunneled requests where it may be leaked.
+        if not scheme.startswith('https') and username and password:
             headers["Proxy-Authorization"] = _basic_auth_str(username, password)
 
         return new_proxies
diff --git a/tests/test_requests.py b/tests/test_requests.py
index b1c8dd4534..b420c44d73 100644
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -647,6 +647,26 @@ def test_proxy_authorization_preserved_on_request(self, httpbin):
 
         assert sent_headers.get("Proxy-Authorization") == proxy_auth_value
 
+
+    @pytest.mark.parametrize(
+        "url,has_proxy_auth",
+        (
+            ('http://example.com', True),
+            ('https://example.com', False),
+        ),
+    )
+    def test_proxy_authorization_not_appended_to_https_request(self, url, has_proxy_auth):
+        session = requests.Session()
+        proxies = {
+            'http': 'http://test:pass@localhost:8080',
+            'https': 'http://test:pass@localhost:8090',
+        }
+        req = requests.Request('GET', url)
+        prep = req.prepare()
+        session.rebuild_proxies(prep, proxies)
+
+        assert ('Proxy-Authorization' in prep.headers) is has_proxy_auth
+
     def test_basicauth_with_netrc(self, httpbin):
         auth = ("user", "pass")
         wrong_auth = ("wronguser", "wrongpass")