diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3369b1015..0fe21aadd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,7 +5,7 @@
 * Support proxy authentication for SOCKS v5 mode (@starplanet)
 * Make it possible to ignore connections in the tls_clienthello event hook (@mhils)
 * Add `tls_established/failed_client/server` event hooks to record negotiation success/failure (@mhils)
-* fix some responses not being decoded properly if the encoding was uppercase #4735 (@Mattwmaster58)
+* fix some responses not being decoded properly if the encoding was uppercase (#4735, @Mattwmaster58)
 * Trigger event hooks for flows with semantically invalid requests, for example invalid content-length headers (@mhils)
 * Improve error message on TLS version mismatch (@mhils)
 * Windows: Switch to Python's default asyncio event loop, which increases the number of sockets
@@ -29,11 +29,12 @@
 * Change connection event hooks to be blocking.
   Processing will only resume once the event hook has finished. (@Prinzhorn)
 * Allow addon hooks to be async (@nneonneo, #4207)
-* Reintroduce `Flow.live`, which signals if a flow belongs to a currently active connection. (@mhils, #4207)
+* Reintroduce `Flow.live`, which signals if a flow belongs to a currently active connection. (#4207, @mhils)
 * Speculative fix for some rare HTTP/2 connection stalls (#5158, @EndUser509)
 * Add ability to specify custom ports with LDAP authentication (#5068, @demonoidvk)
 * Console Improvements on Windows (@mhils)
 * Fix processing of `--set` options (#5067, @marwinxxii) 
+* Lowercase user-added header names and emit a log message to notify the user when using HTTP/2 (#4746, @mhils)
 
 ## 28 September 2021: mitmproxy 7.0.4
 
diff --git a/mitmproxy/addons/proxyserver.py b/mitmproxy/addons/proxyserver.py
index a05fd69c9..06efaf974 100644
--- a/mitmproxy/addons/proxyserver.py
+++ b/mitmproxy/addons/proxyserver.py
@@ -90,6 +90,14 @@ class Proxyserver:
             "proxy_debug", bool, False,
             "Enable debug logs in the proxy core.",
         )
+        loader.add_option(
+            "normalize_outbound_headers", bool, True,
+            """
+            Normalize outgoing HTTP/2 header names, but emit a warning when doing so.
+            HTTP/2 does not allow uppercase header names. This option makes sure that HTTP/2 headers set
+            in custom scripts are lowercased before they are sent.
+            """,
+        )
 
     def running(self):
         self.master = ctx.master
diff --git a/mitmproxy/proxy/layers/http/_http2.py b/mitmproxy/proxy/layers/http/_http2.py
index 8e477c34f..cf71e8425 100644
--- a/mitmproxy/proxy/layers/http/_http2.py
+++ b/mitmproxy/proxy/layers/http/_http2.py
@@ -269,6 +269,13 @@ def normalize_h1_headers(headers: List[Tuple[bytes, bytes]], is_client: bool) ->
     return headers
 
 
+def normalize_h2_headers(headers: List[Tuple[bytes, bytes]]) -> CommandGenerator[None]:
+    for i in range(len(headers)):
+        if not headers[i][0].islower():
+            yield Log(f"Lowercased {repr(headers[i][0]).lstrip('b')} header as uppercase is not allowed with HTTP/2.")
+            headers[i] = (headers[i][0].lower(), headers[i][1])
+
+
 class Http2Server(Http2Connection):
     h2_conf = h2.config.H2Configuration(
         **Http2Connection.h2_conf_defaults,
@@ -290,7 +297,10 @@ class Http2Server(Http2Connection):
                     (b":status", b"%d" % event.response.status_code),
                     *event.response.headers.fields
                 ]
-                if not event.response.is_http2:
+                if event.response.is_http2:
+                    if self.context.options.normalize_outbound_headers:
+                        yield from normalize_h2_headers(headers)
+                else:
                     headers = normalize_h1_headers(headers, False)
 
                 self.h2_conn.send_headers(
@@ -407,6 +417,8 @@ class Http2Client(Http2Connection):
 
             if event.request.is_http2:
                 hdrs = list(event.request.headers.fields)
+                if self.context.options.normalize_outbound_headers:
+                    yield from normalize_h2_headers(hdrs)
             else:
                 headers = event.request.headers
                 if not event.request.authority and "host" in headers:
diff --git a/test/mitmproxy/proxy/layers/http/test_http2.py b/test/mitmproxy/proxy/layers/http/test_http2.py
index 5d76324cc..60ed36369 100644
--- a/test/mitmproxy/proxy/layers/http/test_http2.py
+++ b/test/mitmproxy/proxy/layers/http/test_http2.py
@@ -10,7 +10,7 @@ from mitmproxy.connection import ConnectionState, Server
 from mitmproxy.flow import Error
 from mitmproxy.http import HTTPFlow, Headers, Request
 from mitmproxy.net.http import status_codes
-from mitmproxy.proxy.commands import CloseConnection, OpenConnection, SendData
+from mitmproxy.proxy.commands import CloseConnection, Log, OpenConnection, SendData
 from mitmproxy.proxy.context import Context
 from mitmproxy.proxy.events import ConnectionClosed, DataReceived
 from mitmproxy.proxy.layers import http
@@ -353,19 +353,19 @@ def test_http2_client_aborts(tctx, stream, when, how):
 
 
 @pytest.mark.xfail(reason="inbound validation turned on to protect against request smuggling")
-def test_no_normalization(tctx):
+@pytest.mark.parametrize("normalize", [True, False])
+def test_no_normalization(tctx, normalize):
     """Test that we don't normalize headers when we just pass them through."""
+    tctx.options.normalize_outbound_headers = normalize
 
     server = Placeholder(Server)
     flow = Placeholder(HTTPFlow)
     playbook, cff = start_h2_client(tctx)
 
-    request_headers = example_request_headers + (
-        (b"Should-Not-Be-Capitalized! ", b" :) "),
-    )
-    response_headers = example_response_headers + (
-        (b"Same", b"Here"),
-    )
+    request_headers = list(example_request_headers) + [(b"Should-Not-Be-Capitalized! ", b" :) ")]
+    request_headers_lower = [(k.lower(), v) for (k, v) in request_headers]
+    response_headers = list(example_response_headers) + [(b"Same", b"Here")]
+    response_headers_lower = [(k.lower(), v) for (k, v) in response_headers]
 
     initial = Placeholder(bytes)
     assert (
@@ -385,18 +385,22 @@ def test_no_normalization(tctx):
         hyperframe.frame.SettingsFrame,
         hyperframe.frame.HeadersFrame,
     ]
-    assert hpack.hpack.Decoder().decode(frames[1].data, True) == list(request_headers)
+    assert hpack.hpack.Decoder().decode(frames[1].data, True) == request_headers_lower if normalize else request_headers
 
     sff = FrameFactory()
-    assert (
+    (
             playbook
             >> DataReceived(server, sff.build_headers_frame(response_headers, flags=["END_STREAM"]).serialize())
             << http.HttpResponseHeadersHook(flow)
             >> reply()
             << http.HttpResponseHook(flow)
             >> reply()
-            << SendData(tctx.client, cff.build_headers_frame(response_headers, flags=["END_STREAM"]).serialize())
     )
+    if normalize:
+        playbook << Log("Lowercased 'Same' header as uppercase is not allowed with HTTP/2.")
+    hdrs = response_headers_lower if normalize else response_headers
+    assert playbook << SendData(tctx.client, cff.build_headers_frame(hdrs, flags=["END_STREAM"]).serialize())
+
     assert flow().request.headers.fields == ((b"Should-Not-Be-Capitalized! ", b" :) "),)
     assert flow().response.headers.fields == ((b"Same", b"Here"),)
 
diff --git a/web/src/js/ducks/_options_gen.ts b/web/src/js/ducks/_options_gen.ts
index f077cafc7..56ee7bcaf 100644
--- a/web/src/js/ducks/_options_gen.ts
+++ b/web/src/js/ducks/_options_gen.ts
@@ -34,6 +34,7 @@ export interface OptionsState {
     mode: string
     modify_body: string[]
     modify_headers: string[]
+    normalize_outbound_headers: boolean
     onboarding: boolean
     onboarding_host: string
     onboarding_port: number
@@ -120,6 +121,7 @@ export const defaultState: OptionsState = {
     mode: "regular",
     modify_body: [],
     modify_headers: [],
+    normalize_outbound_headers: true,
     onboarding: true,
     onboarding_host: "mitm.it",
     onboarding_port: 80,