diff --git a/frontend/src/lib/network.ts b/frontend/src/lib/network.ts
index 5976109..e8b43ab 100644
--- a/frontend/src/lib/network.ts
+++ b/frontend/src/lib/network.ts
@@ -10,3 +10,19 @@ export const USDX_ISSUER = import.meta.env.VITE_USDX_ISSUER ?? "";
 export const POOL_ID = import.meta.env.VITE_POOL_ID ?? "";
 
 export const isConfigured = Boolean(CBRL_ISSUER && USDX_ISSUER);
+
+/**
+ * A hard guard, mirroring the backend's `assertTestnet` (src/lib/network.ts): this dApp is
+ * testnet-only. It signs and submits destructive, irreversible ops (freeze, clawback, lock),
+ * so it must never operate against the public network. Returns an error message when the
+ * configured passphrase is PUBLIC so the caller can fail visibly instead of silently signing.
+ */
+export function testnetGuardError(): string | null {
+  if (NETWORK_PASSPHRASE === Networks.PUBLIC) {
+    return (
+      "Stellar Mint is a testnet teaching dApp. It refuses to run against the PUBLIC network. " +
+      "Set VITE_NETWORK_PASSPHRASE to the testnet passphrase."
+    );
+  }
+  return null;
+}
diff --git a/frontend/src/main.tsx b/frontend/src/main.tsx
index 5662951..0861fb0 100644
--- a/frontend/src/main.tsx
+++ b/frontend/src/main.tsx
@@ -3,15 +3,31 @@ import { createRoot } from "react-dom/client";
 import { Buffer } from "buffer";
 import { StoreProvider } from "./store.js";
 import App from "./App.js";
+import { testnetGuardError } from "./lib/network.js";
 import "./index.css";
 
 // stellar-sdk expects Node's Buffer in the browser.
 globalThis.Buffer = globalThis.Buffer ?? Buffer;
 
-createRoot(document.getElementById("root")!).render(
-  <StrictMode>
-    <StoreProvider>
-      <App />
-    </StoreProvider>
-  </StrictMode>,
-);
+const root = createRoot(document.getElementById("root")!);
+
+// Hard guard: testnet-only, mirroring the backend's assertTestnet. Fail visibly before
+// mounting the app so a misconfigured PUBLIC passphrase can never sign a transaction.
+const guardError = testnetGuardError();
+if (guardError) {
+  root.render(
+    <div className="mx-auto max-w-xl px-4 py-8">
+      <div className="rounded-lg border border-danger/40 bg-danger/10 p-4 text-sm">
+        {guardError}
+      </div>
+    </div>,
+  );
+} else {
+  root.render(
+    <StrictMode>
+      <StoreProvider>
+        <App />
+      </StoreProvider>
+    </StrictMode>,
+  );
+}