diff --git a/src/contentScript/contentScript.ts b/src/contentScript/contentScript.ts
index c4069f2fa2..0dde116106 100644
--- a/src/contentScript/contentScript.ts
+++ b/src/contentScript/contentScript.ts
@@ -16,6 +16,7 @@
  */
 
 import "./contentScript.scss";
+import "@/development/visualInjection";
 import { uuidv4 } from "@/types/helpers";
 import {
   isInstalledInThisSession,
diff --git a/src/development/visualInjection.ts b/src/development/visualInjection.ts
new file mode 100644
index 0000000000..514db4685b
--- /dev/null
+++ b/src/development/visualInjection.ts
@@ -0,0 +1,40 @@
+/*
+ * Copyright (C) 2023 PixieBrix, Inc.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+import { MAX_Z_INDEX } from "@/common";
+
+if (process.env.ENVIRONMENT === "development") {
+  const indicator = document.createElement("div");
+
+  // Hide on hover
+  indicator.addEventListener("mouseenter", indicator.remove);
+
+  Object.assign(indicator.style, {
+    position: "fixed",
+    top: 0,
+    height: "1px",
+    zIndex: MAX_Z_INDEX,
+
+    // Vary position to see multiple injections
+    left: `${Math.random() * 100}px`,
+
+    // Add contrast so it's visible no matter the background
+    borderLeft: "solid 5px white",
+    borderRight: "solid 5px black",
+  });
+  document.body.prepend(indicator);
+}