diff --git a/changelog.d/7730.bugfix b/changelog.d/7730.bugfix
new file mode 100644
index 0000000000000000000000000000000000000000..9da254b56cc3f66ec7e8849d0d566afd986431a9
--- /dev/null
+++ b/changelog.d/7730.bugfix
@@ -0,0 +1 @@
+Fix missing `Content-Length` on HTTP responses from the metrics handler.
diff --git a/synapse/metrics/_exposition.py b/synapse/metrics/_exposition.py
index ab7f948ed4538d4a897106a715c8bd05b3a07b42..4304c60d56b933b3cfd365dbb8dd19cb30b9f9c8 100644
--- a/synapse/metrics/_exposition.py
+++ b/synapse/metrics/_exposition.py
@@ -208,6 +208,7 @@ class MetricsHandler(BaseHTTPRequestHandler):
             raise
         self.send_response(200)
         self.send_header("Content-Type", CONTENT_TYPE_LATEST)
+        self.send_header("Content-Length", str(len(output)))
         self.end_headers()
         self.wfile.write(output)
 
@@ -261,4 +262,6 @@ class MetricsResource(Resource):
 
     def render_GET(self, request):
         request.setHeader(b"Content-Type", CONTENT_TYPE_LATEST.encode("ascii"))
-        return generate_latest(self.registry)
+        response = generate_latest(self.registry)
+        request.setHeader(b"Content-Length", str(len(response)))
+        return response