diff --git a/homeassistant/components/rest/data.py b/homeassistant/components/rest/data.py
index b3f35d8802e1..824e54f137ad 100644
--- a/homeassistant/components/rest/data.py
+++ b/homeassistant/components/rest/data.py
@@ -5,7 +5,7 @@ from typing import Any
 
 import aiohttp
 from aiohttp import hdrs
-from multidict import CIMultiDictProxy
+from multidict import CIMultiDict, CIMultiDictProxy
 import xmltodict
 
 from homeassistant.core import HomeAssistant
@@ -30,7 +30,7 @@ class RestData:
         method: str,
         resource: str,
         encoding: str,
-        auth: aiohttp.DigestAuthMiddleware | aiohttp.BasicAuth | tuple[str, str] | None,
+        auth: aiohttp.DigestAuthMiddleware | tuple[str, str] | None,
         headers: dict[str, str] | None,
         params: dict[str, str] | None,
         data: str | None,
@@ -45,13 +45,13 @@ class RestData:
         self._encoding = encoding
         self._force_use_set_encoding = False
 
-        # Convert auth tuple to aiohttp.BasicAuth if needed
+        # Convert an auth tuple to a basic Authorization header if needed
+        self._basic_auth: str | None = None
+        self._digest_auth: aiohttp.DigestAuthMiddleware | None = None
         if isinstance(auth, tuple) and len(auth) == 2:
-            self._auth: aiohttp.BasicAuth | aiohttp.DigestAuthMiddleware | None = (
-                aiohttp.BasicAuth(auth[0], auth[1], encoding="utf-8")
-            )
-        else:
-            self._auth = auth
+            self._basic_auth = aiohttp.encode_basic_auth(auth[0], auth[1])
+        elif isinstance(auth, aiohttp.DigestAuthMiddleware):
+            self._digest_auth = auth
 
         self._headers = headers
         self._params = params
@@ -137,11 +137,14 @@ class RestData:
             "timeout": self._timeout,
         }
 
-        # Handle authentication
-        if isinstance(self._auth, aiohttp.BasicAuth):
-            request_kwargs["auth"] = self._auth
-        elif isinstance(self._auth, aiohttp.DigestAuthMiddleware):
-            request_kwargs["middlewares"] = (self._auth,)
+        # Handle authentication. A configured Authorization header wins,
+        # whatever its casing, so setdefault runs on a CIMultiDict.
+        if self._basic_auth is not None:
+            headers = CIMultiDict(rendered_headers or {})
+            headers.setdefault(hdrs.AUTHORIZATION, self._basic_auth)
+            request_kwargs["headers"] = headers
+        elif self._digest_auth is not None:
+            request_kwargs["middlewares"] = (self._digest_auth,)
 
         # Handle data/content
         if self._request_data:
diff --git a/tests/components/rest/test_data.py b/tests/components/rest/test_data.py
index 9dd1b0fcec2b..9c9fde5ebcbb 100644
--- a/tests/components/rest/test_data.py
+++ b/tests/components/rest/test_data.py
@@ -4,7 +4,9 @@ from datetime import timedelta
 import logging
 from unittest.mock import patch
 
+from aiohttp import hdrs
 from freezegun.api import FrozenDateTimeFactory
+from multidict import CIMultiDict
 import pytest
 
 from homeassistant.components.rest import DOMAIN
@@ -551,3 +553,51 @@ async def test_rest_data_boolean_params_converted_to_strings(
     assert url.query["boolFalse"] == "false"
     assert url.query["stringParam"] == "test"
     assert url.query["intParam"] == "123"
+
+
+@pytest.mark.parametrize(
+    "header_name",
+    [
+        pytest.param("Authorization", id="canonical_casing"),
+        pytest.param("authorization", id="lowercase"),
+    ],
+)
+async def test_rest_data_configured_authorization_header_wins(
+    hass: HomeAssistant,
+    aioclient_mock: AiohttpClientMocker,
+    header_name: str,
+) -> None:
+    """Test a configured Authorization header replaces generated basic auth."""
+    aioclient_mock.get(
+        "http://example.com/api",
+        status=200,
+        json={"status": "ok"},
+        headers={"Content-Type": "application/json"},
+    )
+
+    assert await async_setup_component(
+        hass,
+        DOMAIN,
+        {
+            DOMAIN: {
+                "resource": "http://example.com/api",
+                "method": "GET",
+                "username": "user",
+                "password": "pass",
+                "headers": {header_name: "Bearer configured"},
+                "sensor": [
+                    {
+                        "name": "test_sensor",
+                        "value_template": "{{ value_json.status }}",
+                    }
+                ],
+            }
+        },
+    )
+    await hass.async_block_till_done()
+
+    assert len(aioclient_mock.mock_calls) == 1
+    _method, _url, _data, headers = aioclient_mock.mock_calls[0]
+
+    # The generated basic auth must not be sent as a second Authorization header
+    assert CIMultiDict(headers).getall(hdrs.AUTHORIZATION) == ["Bearer configured"]