diff --git a/src/twisted/web/http_headers.py b/src/twisted/web/http_headers.py
index 738a1759c6b..f810f4bc2c4 100644
--- a/src/twisted/web/http_headers.py
+++ b/src/twisted/web/http_headers.py
@@ -18,7 +18,6 @@
     Tuple,
     TypeVar,
     Union,
-    cast,
     overload,
 )
 
@@ -85,7 +84,7 @@ class Headers:
     def __init__(
         self,
         rawHeaders: Optional[Mapping[AnyStr, Sequence[AnyStr]]] = None,
-    ):
+    ) -> None:
         self._rawHeaders: Dict[bytes, List[bytes]] = {}
         if rawHeaders is not None:
             for name, values in rawHeaders.items():
@@ -111,7 +110,7 @@ def __cmp__(self, other):
             )
         return NotImplemented
 
-    def _encodeName(self, name: AnyStr) -> bytes:
+    def _encodeName(self, name: Union[str, bytes]) -> bytes:
         """
         Encode the name of a header (eg 'Content-Type') to an ISO-8859-1 encoded
         bytestring if required.
@@ -152,7 +151,21 @@ def removeHeader(self, name: AnyStr) -> None:
         """
         self._rawHeaders.pop(self._encodeName(name), None)
 
-    def setRawHeaders(self, name: AnyStr, values: Sequence[AnyStr]) -> None:
+    @overload
+    def setRawHeaders(self, name: Union[str, bytes], values: Sequence[bytes]) -> None:
+        ...
+
+    @overload
+    def setRawHeaders(self, name: Union[str, bytes], values: Sequence[str]) -> None:
+        ...
+
+    @overload
+    def setRawHeaders(
+        self, name: Union[str, bytes], values: Sequence[Union[str, bytes]]
+    ) -> None:
+        ...
+
+    def setRawHeaders(self, name: Union[str, bytes], values: object) -> None:
         """
         Sets the raw representation of the given header.
 
@@ -161,9 +174,8 @@ def setRawHeaders(self, name: AnyStr, values: Sequence[AnyStr]) -> None:
         @param values: A list of strings each one being a header value of
             the given name.
 
-        @raise TypeError: Raised if C{values} is not a L{list} of L{bytes}
-            or L{str} strings, or if C{name} is not a L{bytes} or
-            L{str} string.
+        @raise TypeError: Raised if C{values} is not a sequence of L{bytes}
+            or L{str}, or if C{name} is not L{bytes} or L{str}.
 
         @return: L{None}
         """
@@ -175,7 +187,7 @@ def setRawHeaders(self, name: AnyStr, values: Sequence[AnyStr]) -> None:
 
         if not isinstance(name, (bytes, str)):
             raise TypeError(
-                "Header name is an instance of %r, " "not bytes or str" % (type(name),)
+                f"Header name is an instance of {type(name)!r}, not bytes or str"
             )
 
         for count, value in enumerate(values):
@@ -200,7 +212,7 @@ def setRawHeaders(self, name: AnyStr, values: Sequence[AnyStr]) -> None:
 
         self._rawHeaders[_name] = encodedValues
 
-    def addRawHeader(self, name: AnyStr, value: AnyStr) -> None:
+    def addRawHeader(self, name: Union[str, bytes], value: Union[str, bytes]) -> None:
         """
         Add a new raw value for the given header.
 
@@ -210,7 +222,7 @@ def addRawHeader(self, name: AnyStr, value: AnyStr) -> None:
         """
         if not isinstance(name, (bytes, str)):
             raise TypeError(
-                "Header name is an instance of %r, " "not bytes or str" % (type(name),)
+                f"Header name is an instance of {type(name)!r}, not bytes or str"
             )
 
         if not isinstance(value, (bytes, str)):
@@ -219,11 +231,13 @@ def addRawHeader(self, name: AnyStr, value: AnyStr) -> None:
                 "bytes or str" % (type(value),)
             )
 
-        # We secretly know getRawHeaders is really returning a list
-        values = cast(List[AnyStr], self.getRawHeaders(name, default=[]))
-        values.append(value)
-
-        self.setRawHeaders(name, values)
+        self._rawHeaders.setdefault(
+            _sanitizeLinearWhitespace(self._encodeName(name)), []
+        ).append(
+            _sanitizeLinearWhitespace(
+                value.encode("utf8") if isinstance(value, str) else value
+            )
+        )
 
     @overload
     def getRawHeaders(self, name: AnyStr) -> Optional[Sequence[AnyStr]]:
diff --git a/src/twisted/web/newsfragments/11635.bugfix b/src/twisted/web/newsfragments/11635.bugfix
new file mode 100644
index 00000000000..a32e51e7618
--- /dev/null
+++ b/src/twisted/web/newsfragments/11635.bugfix
@@ -0,0 +1 @@
+The typing of the twisted.web.http_headers.Headers methods addRawHeader() and setRawHeaders() now allow mixing str and bytes, matching the runtime behavior.
diff --git a/src/twisted/web/test/test_http_headers.py b/src/twisted/web/test/test_http_headers.py
index 4f9f71f125f..1e039fd78b6 100644
--- a/src/twisted/web/test/test_http_headers.py
+++ b/src/twisted/web/test/test_http_headers.py
@@ -668,3 +668,36 @@ def test_copy(self):
         # Verify that the orignal does not have it
         self.assertEqual(h.getRawHeaders("test\u00E1"), ["foo\u2603", "bar"])
         self.assertEqual(h.getRawHeaders(b"test\xe1"), [b"foo\xe2\x98\x83", b"bar"])
+
+
+class MixedHeadersTests(TestCase):
+    """
+    Tests for L{Headers}, mixing L{bytes} and L{str} arguments for methods
+    where that is permitted.
+    """
+
+    def test_addRawHeader(self) -> None:
+        """
+        L{Headers.addRawHeader} accepts mixed L{str} and L{bytes}.
+        """
+        h = Headers()
+        h.addRawHeader(b"bytes", "str")
+        h.addRawHeader("str", b"bytes")
+
+        self.assertEqual(h.getRawHeaders(b"Bytes"), [b"str"])
+        self.assertEqual(h.getRawHeaders("Str"), ["bytes"])
+
+    def test_setRawHeaders(self) -> None:
+        """
+        L{Headers.setRawHeaders} accepts mixed L{str} and L{bytes}.
+        """
+        h = Headers()
+        h.setRawHeaders(b"bytes", [b"bytes"])
+        h.setRawHeaders("str", ["str"])
+        h.setRawHeaders("mixed-str", [b"bytes", "str"])
+        h.setRawHeaders(b"mixed-bytes", ["str", b"bytes"])
+
+        self.assertEqual(h.getRawHeaders(b"Bytes"), [b"bytes"])
+        self.assertEqual(h.getRawHeaders("Str"), ["str"])
+        self.assertEqual(h.getRawHeaders("Mixed-Str"), ["bytes", "str"])
+        self.assertEqual(h.getRawHeaders(b"Mixed-Bytes"), [b"str", b"bytes"])