diff --git a/modules/caddyhttp/reverseproxy/reverseproxy.go b/modules/caddyhttp/reverseproxy/reverseproxy.go
index f062ef5988b..91c651bd17b 100644
--- a/modules/caddyhttp/reverseproxy/reverseproxy.go
+++ b/modules/caddyhttp/reverseproxy/reverseproxy.go
@@ -695,6 +695,7 @@ func (h *Handler) proxyLoopIteration(r *http.Request, origReq *http.Request, w h
 		if userOps != nil {
 			userOps.ApplyToRequest(r)
 		}
+		normalizeWebsocketHeaders(r.Header)
 	}
 
 	// proxy the request to that upstream
diff --git a/modules/caddyhttp/reverseproxy/websocket_test.go b/modules/caddyhttp/reverseproxy/websocket_test.go
new file mode 100644
index 00000000000..fc626c2eebd
--- /dev/null
+++ b/modules/caddyhttp/reverseproxy/websocket_test.go
@@ -0,0 +1,109 @@
+package reverseproxy
+
+import (
+	"net/http"
+	"testing"
+)
+
+func TestNormalizeWebsocketHeaders(t *testing.T) {
+	tests := []struct {
+		name   string
+		input  http.Header
+		want   http.Header
+	}{
+		{
+			name: "canonicalized headers are renamed to RFC 6455 form",
+			input: http.Header{
+				// Go's http.CanonicalHeaderKey lowercases the 'S' in WebSocket:
+				// "Sec-WebSocket-Key" -> "Sec-Websocket-Key"
+				"Sec-Websocket-Key":        {"dGhlIHNhbXBsZSBub25jZQ=="},
+				"Sec-Websocket-Version":    {"13"},
+				"Sec-Websocket-Protocol":   {"chat"},
+				"Sec-Websocket-Extensions": {"permessage-deflate"},
+			},
+			want: http.Header{
+				"Sec-WebSocket-Key":        {"dGhlIHNhbXBsZSBub25jZQ=="},
+				"Sec-WebSocket-Version":    {"13"},
+				"Sec-WebSocket-Protocol":   {"chat"},
+				"Sec-WebSocket-Extensions": {"permessage-deflate"},
+			},
+		},
+		{
+			name: "already-correct headers are left unchanged",
+			input: http.Header{
+				"Sec-WebSocket-Key":     {"abc123"},
+				"Sec-WebSocket-Version": {"13"},
+			},
+			want: http.Header{
+				"Sec-WebSocket-Key":     {"abc123"},
+				"Sec-WebSocket-Version": {"13"},
+			},
+		},
+		{
+			name:  "non-WebSocket headers are untouched",
+			input: http.Header{"Content-Type": {"text/plain"}, "X-Foo": {"bar"}},
+			want:  http.Header{"Content-Type": {"text/plain"}, "X-Foo": {"bar"}},
+		},
+		{
+			name:  "empty header map is a no-op",
+			input: http.Header{},
+			want:  http.Header{},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			normalizeWebsocketHeaders(tt.input)
+			for k, wantV := range tt.want {
+				gotV, ok := tt.input[k]
+				if !ok {
+					t.Errorf("missing header %q", k)
+					continue
+				}
+				if len(gotV) != len(wantV) || gotV[0] != wantV[0] {
+					t.Errorf("header %q: got %v, want %v", k, gotV, wantV)
+				}
+			}
+			// Ensure no extra keys remain (old canonical forms must be deleted).
+			for k := range tt.input {
+				if _, ok := tt.want[k]; !ok {
+					t.Errorf("unexpected header key left in map: %q", k)
+				}
+			}
+		})
+	}
+}
+
+// TestNormalizeWebsocketHeadersSurvivesCopyHeader is a regression test for
+// https://github.com/caddyserver/caddy/issues/7784.
+//
+// proxyLoopIteration rebuilds r.Header with copyHeader when transport or header
+// ops are configured. copyHeader uses http.Header.Add internally, which calls
+// http.CanonicalHeaderKey and lowercases the 'S' in "WebSocket" to produce
+// "Sec-Websocket-*". The fix calls normalizeWebsocketHeaders after the rebuild
+// so the RFC 6455 casing is restored before the request is forwarded.
+func TestNormalizeWebsocketHeadersSurvivesCopyHeader(t *testing.T) {
+	// Simulate the state of r.Header after copyHeader re-canonicalizes it.
+	rebuilt := make(http.Header)
+	// http.Header.Add canonicalizes to "Sec-Websocket-Key" (lowercase 's').
+	rebuilt.Add("Sec-WebSocket-Key", "dGhlIHNhbXBsZSBub25jZQ==")
+	rebuilt.Add("Sec-WebSocket-Version", "13")
+
+	// At this point the map contains the lowercase form.
+	if _, ok := rebuilt["Sec-Websocket-Key"]; !ok {
+		t.Fatal("test setup: expected canonical (lowercase) key to be present after Add")
+	}
+
+	// The fix: call normalizeWebsocketHeaders after the rebuild.
+	normalizeWebsocketHeaders(rebuilt)
+
+	// RFC 6455 form must be present (direct map lookup — .Get() re-canonicalizes
+	// to "Sec-Websocket-Key" and would miss the corrected key).
+	if _, ok := rebuilt["Sec-WebSocket-Key"]; !ok {
+		t.Error("Sec-WebSocket-Key missing after normalize; WebSocket upgrade will fail")
+	}
+	// Lowercase form must be gone.
+	if _, ok := rebuilt["Sec-Websocket-Key"]; ok {
+		t.Error("canonical (lowercase) Sec-Websocket-Key still present after normalize")
+	}
+}