diff --git a/tests/test_auth.py b/tests/test_auth.py
index f2c7da6..fcd266e 100644
--- a/tests/test_auth.py
+++ b/tests/test_auth.py
@@ -1,6 +1,9 @@
+from base64 import b64encode
+from contextlib import asynccontextmanager
 from os import environ
 from subprocess import run
 from tempfile import NamedTemporaryFile
+from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
@@ -133,3 +136,28 @@ def test_precedence(scenario, expected, monkeypatch):
             match="api_key is not available when using an Ethereum private key",
         ):
             client.api_key  # noqa: B018
+
+
+@pytest.mark.asyncio
+async def test_basic_auth_header():
+    api_key = "testkey"
+    captured_headers = {}
+
+    response_mock = MagicMock()
+    response_mock.status = 200
+    response_mock.headers = {}
+    response_mock.__aenter__ = AsyncMock(return_value=response_mock)
+    response_mock.__aexit__ = AsyncMock(return_value=False)
+    response_mock.json = AsyncMock(return_value={"url": "https://a.example"})
+
+    @asynccontextmanager
+    async def fake_post(**kwargs):
+        captured_headers.update(kwargs.get("headers", {}))
+        yield response_mock
+
+    with patch("zyte_api._async._post_func", return_value=fake_post):
+        client = AsyncZyteAPI(api_key=api_key)
+        await client.get({"url": "https://a.example", "httpResponseBody": True})
+
+    expected = "Basic " + b64encode(f"{api_key}:".encode()).decode()
+    assert captured_headers.get("Authorization") == expected
diff --git a/zyte_api/_async.py b/zyte_api/_async.py
index 39588d4..69804b1 100644
--- a/zyte_api/_async.py
+++ b/zyte_api/_async.py
@@ -203,9 +203,11 @@ async def get(
         query = _process_query(query)
         headers = {"User-Agent": self.user_agent, "Accept-Encoding": "br"}
 
-        auth_kwargs = {}
         if isinstance(self._auth, str):
-            auth_kwargs["auth"] = aiohttp.BasicAuth(self._auth)
+            if hasattr(aiohttp, "encode_basic_auth"):  # aiohttp 3.14+
+                headers["Authorization"] = aiohttp.encode_basic_auth(self._auth, "")
+            else:
+                headers["Authorization"] = aiohttp.BasicAuth(self._auth).encode()
         else:
             x402_headers = await self._auth.get_headers(url, query, headers, post)
             headers.update(x402_headers)
@@ -214,7 +216,6 @@ async def get(
             "url": url,
             "json": query,
             "headers": headers,
-            **auth_kwargs,
         }
 
         response_stats = []