diff --git a/README.md b/README.md
index ef91e04..ab1ff6b 100644
--- a/README.md
+++ b/README.md
@@ -418,6 +418,10 @@ Each group of these assertions records a coverage key and a final case demands
 the exact set, so a section that silently stopped running reddens the suite
 instead of quietly shrinking it.
 
+It also covers the red banner on the Settings screen: switching the network
+there to Sepolia must put up the `[TESTNET]` banner, and switching back to
+mainnet must take it down, both while Settings is still on screen.
+
 It also covers the confirmation screen, for both a native ETH send and an ERC-20
 send: Send disabled while the fee estimate is in flight, enabled once it lands,
 the fee block quoting the expected cost and the reserve separately, the distinct
diff --git a/src/popup/views/helpers.js b/src/popup/views/helpers.js
index dba54d6..f2585c2 100644
--- a/src/popup/views/helpers.js
+++ b/src/popup/views/helpers.js
@@ -128,7 +128,8 @@ function showView(name) {
 }
 
 // Create or update the debug/insecure warning banner.
-// Called on every view switch and after the settings debug toggle changes.
+// Called on every view switch, after the settings debug toggle changes, and
+// after a network switch in Settings.
 // The banner is shown when the compile-time DEBUG constant is true OR when
 // the user has enabled runtime debug mode via the settings easter egg, OR
 // when the active network is a testnet.
diff --git a/src/popup/views/settings.js b/src/popup/views/settings.js
index cefde1c..c5a727d 100644
--- a/src/popup/views/settings.js
+++ b/src/popup/views/settings.js
@@ -343,11 +343,13 @@ function init(ctx) {
         // switch request.
         notify({ type: "AUTISTMASK_NETWORK_CHANGED", chainId: net.chainId });
         resetRenderedViews();
-        // Settings itself is brought up to date for the new network here.
+        // Settings itself, and the testnet banner above it, are brought up
+        // to date for the new network here.
         markViewRendered("settings");
         $("settings-rpc").value = state.rpcUrl;
         $("settings-blockscout").value = state.blockscoutUrl;
         renderTrackedTokens();
+        updateDebugBanner(state.currentView);
         showFlash("Switched to " + net.name + ".");
     });
 
diff --git a/tests/e2e/run.js b/tests/e2e/run.js
index 24d2aec..2fdd9ad 100644
--- a/tests/e2e/run.js
+++ b/tests/e2e/run.js
@@ -1824,6 +1824,65 @@ test("the Settings assertions above all ran (#229)", async (env) => {
     );
 });
 
+// ------------------------------------------------ testnet banner (#511)
+
+// This suite runs a release build, so the red banner at the top reads
+// "[TESTNET]" on Sepolia and is absent on mainnet. A network switch in
+// Settings must change it while Settings is still on screen; it used to
+// change only when the next screen came up
+// (https://git.eeqj.de/sneak/AutistMask/issues/511).
+
+// The banner's text, or null when there is none, once the switch to `net`
+// has finished. The switch's flash is the last thing its handler does, and
+// showView() clears the flash, so the banner read while that flash is up is
+// the one the switch left on the screen it was made from.
+async function bannerAfterSwitch(page, net) {
+    await page.waitForFunction(
+        (text) => document.getElementById("flash-msg").textContent === text,
+        "Switched to " + net.name + ".",
+    );
+    return page.evaluate(() => {
+        const banner = document.getElementById("debug-banner");
+        return {
+            text: banner === null ? null : banner.textContent,
+            onSettings: !document
+                .getElementById("view-settings")
+                .classList.contains("hidden"),
+        };
+    });
+}
+
+test("a network switch in Settings shows and removes the testnet banner at once (#511)", async (env) => {
+    const { mainnet, sepolia } = NETWORKS;
+    await openSettings(env.page);
+    assert(
+        (await env.page.locator("#debug-banner").count()) === 0,
+        "a banner is up on mainnet before the switch",
+    );
+
+    await env.page.selectOption("#settings-network", "sepolia");
+    let after = await bannerAfterSwitch(env.page, sepolia);
+    assert(after.onSettings, "Settings left the screen on the switch");
+    assert(
+        after.text === "[TESTNET]",
+        "after the switch to Sepolia, Settings shows the banner " +
+            JSON.stringify(after.text) +
+            ', expected "[TESTNET]"',
+    );
+
+    await env.page.selectOption("#settings-network", "mainnet");
+    after = await bannerAfterSwitch(env.page, mainnet);
+    assert(after.onSettings, "Settings left the screen on the switch back");
+    assert(
+        after.text === null,
+        "after the switch back to mainnet, Settings still shows the banner " +
+            JSON.stringify(after.text),
+    );
+
+    await env.page.click("#btn-settings-back");
+    await visible(env.page, "#view-main");
+});
+
 // ------------------------------------------------ dust threshold (#233)
 
 // The popup size README documents the UI as designed for. Pages in this