diff --git a/django/http/request.py b/django/http/request.py
index 86ea68c20c..fe15a173be 100644
--- a/django/http/request.py
+++ b/django/http/request.py
@@ -7,6 +7,7 @@ from urllib.parse import parse_qsl, quote, urlencode, urljoin, urlsplit
 from django.conf import settings
 from django.core import signing
 from django.core.exceptions import (
+    BadRequest,
     DisallowedHost,
     ImproperlyConfigured,
     RequestDataTooBig,
@@ -377,10 +378,16 @@ class HttpRequest:
                 self._mark_post_parse_error()
                 raise
         elif self.content_type == "application/x-www-form-urlencoded":
-            self._post, self._files = (
-                QueryDict(self.body, encoding=self._encoding),
-                MultiValueDict(),
-            )
+            # According to RFC 1866, the "application/x-www-form-urlencoded"
+            # content type does not have a charset and should be always treated
+            # as UTF-8.
+            if self._encoding is not None and self._encoding.lower() != "utf-8":
+                raise BadRequest(
+                    "HTTP requests with the 'application/x-www-form-urlencoded' "
+                    "content type must be UTF-8 encoded."
+                )
+            self._post = QueryDict(self.body, encoding="utf-8")
+            self._files = MultiValueDict()
         else:
             self._post, self._files = (
                 QueryDict(encoding=self._encoding),
diff --git a/docs/releases/5.0.txt b/docs/releases/5.0.txt
index 0b65ee189a..7379289e42 100644
--- a/docs/releases/5.0.txt
+++ b/docs/releases/5.0.txt
@@ -580,6 +580,10 @@ Miscellaneous
 * Executing SQL queries before the app registry has been fully populated now
   raises :exc:`RuntimeWarning`.
 
+* :exc:`~django.core.exceptions.BadRequest` is raised for non-UTF-8 encoded
+  requests with the :mimetype:`application/x-www-form-urlencoded` content type.
+  See :rfc:`1866` for more details.
+
 .. _deprecated-features-5.0:
 
 Features deprecated in 5.0
diff --git a/tests/requests_tests/tests.py b/tests/requests_tests/tests.py
index 099f600867..228198ae8a 100644
--- a/tests/requests_tests/tests.py
+++ b/tests/requests_tests/tests.py
@@ -3,7 +3,7 @@ from io import BytesIO
 from itertools import chain
 from urllib.parse import urlencode
 
-from django.core.exceptions import DisallowedHost
+from django.core.exceptions import BadRequest, DisallowedHost
 from django.core.handlers.wsgi import LimitedStream, WSGIRequest
 from django.http import (
     HttpHeaders,
@@ -369,10 +369,7 @@ class RequestsTests(SimpleTestCase):
         )
         self.assertEqual(request.POST, {"key": ["España"]})
 
-    def test_alternate_charset_POST(self):
-        """
-        Test a POST with non-utf-8 payload encoding.
-        """
+    def test_non_utf8_charset_POST_bad_request(self):
         payload = FakePayload(urlencode({"key": "España".encode("latin-1")}))
         request = WSGIRequest(
             {
@@ -382,7 +379,30 @@ class RequestsTests(SimpleTestCase):
                 "wsgi.input": payload,
             }
         )
-        self.assertEqual(request.POST, {"key": ["España"]})
+        msg = (
+            "HTTP requests with the 'application/x-www-form-urlencoded' content type "
+            "must be UTF-8 encoded."
+        )
+        with self.assertRaisesMessage(BadRequest, msg):
+            request.POST
+        with self.assertRaisesMessage(BadRequest, msg):
+            request.FILES
+
+    def test_utf8_charset_POST(self):
+        for charset in ["utf-8", "UTF-8"]:
+            with self.subTest(charset=charset):
+                payload = FakePayload(urlencode({"key": "España"}))
+                request = WSGIRequest(
+                    {
+                        "REQUEST_METHOD": "POST",
+                        "CONTENT_LENGTH": len(payload),
+                        "CONTENT_TYPE": (
+                            f"application/x-www-form-urlencoded; charset={charset}"
+                        ),
+                        "wsgi.input": payload,
+                    }
+                )
+                self.assertEqual(request.POST, {"key": ["España"]})
 
     def test_body_after_POST_multipart_form_data(self):
         """
@@ -694,18 +714,31 @@ class RequestsTests(SimpleTestCase):
             request.body
 
     def test_set_encoding_clears_POST(self):
-        payload = FakePayload("name=Hello Günter")
+        payload = FakePayload(
+            "\r\n".join(
+                [
+                    f"--{BOUNDARY}",
+                    'Content-Disposition: form-data; name="name"',
+                    "",
+                    "Hello Günter",
+                    f"--{BOUNDARY}--",
+                    "",
+                ]
+            )
+        )
         request = WSGIRequest(
             {
                 "REQUEST_METHOD": "POST",
-                "CONTENT_TYPE": "application/x-www-form-urlencoded",
+                "CONTENT_TYPE": MULTIPART_CONTENT,
                 "CONTENT_LENGTH": len(payload),
                 "wsgi.input": payload,
             }
         )
         self.assertEqual(request.POST, {"name": ["Hello Günter"]})
         request.encoding = "iso-8859-16"
-        self.assertEqual(request.POST, {"name": ["Hello GĂŒnter"]})
+        # FIXME: POST should be accessible after changing the encoding
+        # (refs #14035).
+        # self.assertEqual(request.POST, {"name": ["Hello GĂŒnter"]})
 
     def test_set_encoding_clears_GET(self):
         payload = FakePayload("")