diff --git a/modules/caddyhttp/reverseproxy/reverseproxy.go b/modules/caddyhttp/reverseproxy/reverseproxy.go
index bcbc1ff4..123bf774 100644
--- a/modules/caddyhttp/reverseproxy/reverseproxy.go
+++ b/modules/caddyhttp/reverseproxy/reverseproxy.go
@@ -569,6 +569,30 @@ func (h *Handler) proxyLoopIteration(r *http.Request, origReq *http.Request, w h
 	return false, proxyErr
 }
 
+// Mapping of the canonical form of the headers, to the RFC 6455 form,
+// i.e. `WebSocket` with uppercase 'S'.
+var websocketHeaderMapping = map[string]string{
+	"Sec-Websocket-Accept":     "Sec-WebSocket-Accept",
+	"Sec-Websocket-Extensions": "Sec-WebSocket-Extensions",
+	"Sec-Websocket-Key":        "Sec-WebSocket-Key",
+	"Sec-Websocket-Protocol":   "Sec-WebSocket-Protocol",
+	"Sec-Websocket-Version":    "Sec-WebSocket-Version",
+}
+
+// normalizeWebsocketHeaders ensures we use the standard casing as per
+// RFC 6455, i.e. `WebSocket` with uppercase 'S'. Most servers don't
+// care about this difference (read headers case insensitively), but
+// some do, so this maximizes compatibility with upstreams.
+// See https://github.com/caddyserver/caddy/pull/6621
+func normalizeWebsocketHeaders(header http.Header) {
+	for k, rk := range websocketHeaderMapping {
+		if v, ok := header[k]; ok {
+			delete(header, k)
+			header[rk] = v
+		}
+	}
+}
+
 // prepareRequest clones req so that it can be safely modified without
 // changing the original request or introducing data races. It then
 // modifies it so that it is ready to be proxied, except for directing
@@ -655,6 +679,7 @@ func (h Handler) prepareRequest(req *http.Request, repl *caddy.Replacer) (*http.
 	if reqUpType != "" {
 		req.Header.Set("Connection", "Upgrade")
 		req.Header.Set("Upgrade", reqUpType)
+		normalizeWebsocketHeaders(req.Header)
 	}
 
 	// Set up the PROXY protocol info
diff --git a/modules/caddyhttp/reverseproxy/streaming.go b/modules/caddyhttp/reverseproxy/streaming.go
index c871a3fa..3fde10b3 100644
--- a/modules/caddyhttp/reverseproxy/streaming.go
+++ b/modules/caddyhttp/reverseproxy/streaming.go
@@ -66,6 +66,7 @@ func (h *Handler) handleUpgradeResponse(logger *zap.Logger, wg *sync.WaitGroup,
 	// write header first, response headers should not be counted in size
 	// like the rest of handler chain.
 	copyHeader(rw.Header(), res.Header)
+	normalizeWebsocketHeaders(rw.Header())
 	rw.WriteHeader(res.StatusCode)
 
 	logger.Debug("upgrading connection")