diff --git a/changelog.d/6077.misc b/changelog.d/6077.misc
new file mode 100644
index 0000000000..31ac5b97a4
--- /dev/null
+++ b/changelog.d/6077.misc
@@ -0,0 +1 @@
+Edit header dicts docstrings in SimpleHttpClient to note that `str` or `bytes` can be passed as header keys.
diff --git a/synapse/http/client.py b/synapse/http/client.py
index 51765ae3c0..cdf828a4ff 100644
--- a/synapse/http/client.py
+++ b/synapse/http/client.py
@@ -327,7 +327,7 @@ class SimpleHttpClient(object):
         Args:
             uri (str):
             args (dict[str, str|List[str]]): query params
-            headers (dict[str, List[str]]|None): If not None, a map from
+            headers (dict[str|bytes, List[str|bytes]]|None): If not None, a map from
                header name to a list of values for that header
 
         Returns:
@@ -371,7 +371,7 @@ class SimpleHttpClient(object):
         Args:
             uri (str):
             post_json (object):
-            headers (dict[str, List[str]]|None): If not None, a map from
+            headers (dict[str|bytes, List[str|bytes]]|None): If not None, a map from
                header name to a list of values for that header
 
         Returns:
@@ -414,7 +414,7 @@ class SimpleHttpClient(object):
                 None.
                 **Note**: The value of each key is assumed to be an iterable
                 and *not* a string.
-            headers (dict[str, List[str]]|None): If not None, a map from
+            headers (dict[str|bytes, List[str|bytes]]|None): If not None, a map from
                header name to a list of values for that header
         Returns:
             Deferred: Succeeds when we get *any* 2xx HTTP response, with the
@@ -438,7 +438,7 @@ class SimpleHttpClient(object):
                 None.
                 **Note**: The value of each key is assumed to be an iterable
                 and *not* a string.
-            headers (dict[str, List[str]]|None): If not None, a map from
+            headers (dict[str|bytes, List[str|bytes]]|None): If not None, a map from
                header name to a list of values for that header
         Returns:
             Deferred: Succeeds when we get *any* 2xx HTTP response, with the
@@ -482,7 +482,7 @@ class SimpleHttpClient(object):
                 None.
                 **Note**: The value of each key is assumed to be an iterable
                 and *not* a string.
-            headers (dict[str, List[str]]|None): If not None, a map from
+            headers (dict[str|bytes, List[str|bytes]]|None): If not None, a map from
                header name to a list of values for that header
         Returns:
             Deferred: Succeeds when we get *any* 2xx HTTP response, with the
@@ -516,7 +516,7 @@ class SimpleHttpClient(object):
         Args:
             url (str): The URL to GET
             output_stream (file): File to write the response body to.
-            headers (dict[str, List[str]]|None): If not None, a map from
+            headers (dict[str|bytes, List[str|bytes]]|None): If not None, a map from
                header name to a list of values for that header
         Returns:
             A (int,dict,string,int) tuple of the file length, dict of the response