diff --git a/yarn-project/cli-wallet/src/cmds/default_mnemonic_guard.test.ts b/yarn-project/cli-wallet/src/cmds/default_mnemonic_guard.test.ts
new file mode 100644
index 000000000000..269843d894a1
--- /dev/null
+++ b/yarn-project/cli-wallet/src/cmds/default_mnemonic_guard.test.ts
@@ -0,0 +1,38 @@
+import { describe, expect, it } from '@jest/globals';
+
+import { DEFAULT_TEST_MNEMONIC, assertSafeL1Signer } from './default_mnemonic_guard.js';
+
+describe('assertSafeL1Signer', () => {
+  const implicitDefault = {
+    mnemonic: DEFAULT_TEST_MNEMONIC,
+    mnemonicWasExplicit: false,
+    privateKey: undefined,
+    allowDefaultMnemonic: false,
+  };
+
+  it.each([31337, 1337])('allows the implicit test mnemonic on local chain %i', chainId => {
+    expect(() => assertSafeL1Signer({ ...implicitDefault, chainId })).not.toThrow();
+  });
+
+  it.each([1, 11155111, 42])('rejects the implicit test mnemonic on non-local chain %i', chainId => {
+    expect(() => assertSafeL1Signer({ ...implicitDefault, chainId })).toThrow(
+      `--l1-chain-id ${chainId} is not a local network`,
+    );
+  });
+
+  it('allows an explicit private key on a non-local chain', () => {
+    expect(() => assertSafeL1Signer({ ...implicitDefault, chainId: 1, privateKey: '0x1234' })).not.toThrow();
+  });
+
+  it('allows a custom mnemonic on a non-local chain', () => {
+    expect(() => assertSafeL1Signer({ ...implicitDefault, chainId: 1, mnemonic: 'custom mnemonic' })).not.toThrow();
+  });
+
+  it('allows an explicitly supplied test mnemonic on a non-local chain', () => {
+    expect(() => assertSafeL1Signer({ ...implicitDefault, chainId: 1, mnemonicWasExplicit: true })).not.toThrow();
+  });
+
+  it('allows an explicit unsafe opt-in on a non-local chain', () => {
+    expect(() => assertSafeL1Signer({ ...implicitDefault, chainId: 1, allowDefaultMnemonic: true })).not.toThrow();
+  });
+});
diff --git a/yarn-project/cli-wallet/src/cmds/default_mnemonic_guard.ts b/yarn-project/cli-wallet/src/cmds/default_mnemonic_guard.ts
new file mode 100644
index 000000000000..a08cb45b40be
--- /dev/null
+++ b/yarn-project/cli-wallet/src/cmds/default_mnemonic_guard.ts
@@ -0,0 +1,29 @@
+export const DEFAULT_TEST_MNEMONIC = 'test test test test test test test test test test test junk';
+
+const LOCAL_L1_CHAIN_IDS = new Set([1337, 31337]);
+
+export function assertSafeL1Signer({
+  chainId,
+  privateKey,
+  mnemonic,
+  mnemonicWasExplicit,
+  allowDefaultMnemonic,
+}: {
+  chainId: number;
+  privateKey?: string;
+  mnemonic: string;
+  mnemonicWasExplicit: boolean;
+  allowDefaultMnemonic: boolean;
+}) {
+  const usesImplicitTestMnemonic =
+    !privateKey && mnemonic === DEFAULT_TEST_MNEMONIC && !mnemonicWasExplicit && !allowDefaultMnemonic;
+  if (LOCAL_L1_CHAIN_IDS.has(chainId) || !usesImplicitTestMnemonic) {
+    return;
+  }
+
+  throw new Error(
+    `--l1-chain-id ${chainId} is not a local network, but no --l1-private-key or explicit --mnemonic was provided. ` +
+      'Refusing to sign with the public default test mnemonic. Pass --l1-private-key, an explicit --mnemonic, ' +
+      'or --i-know-this-uses-the-public-test-mnemonic to proceed.',
+  );
+}
diff --git a/yarn-project/cli-wallet/src/cmds/index.ts b/yarn-project/cli-wallet/src/cmds/index.ts
index b9d362bc93e0..5e5bc4b1039f 100644
--- a/yarn-project/cli-wallet/src/cmds/index.ts
+++ b/yarn-project/cli-wallet/src/cmds/index.ts
@@ -40,6 +40,7 @@ import {
   createVerboseOption,
   integerArgParser,
 } from '../utils/options/index.js';
+import { DEFAULT_TEST_MNEMONIC, assertSafeL1Signer } from './default_mnemonic_guard.js';
 
 function parseWaitForStatus(status: string): TxStatus {
   switch (status) {
@@ -496,11 +497,16 @@ export function injectCommands(
     .option(
       '-m, --mnemonic <string>',
       'The mnemonic to use for deriving the Ethereum address that will mint and bridge',
-      'test test test test test test test test test test test junk',
+      DEFAULT_TEST_MNEMONIC,
     )
     .option('--mint', 'Mint the tokens on L1', false)
     .option('--l1-private-key <string>', 'The private key to the eth account bridging', PRIVATE_KEY)
     .addOption(l1ChainIdOption)
+    .option(
+      '--i-know-this-uses-the-public-test-mnemonic',
+      'Allow the public default test mnemonic on a non-local L1 network',
+      false,
+    )
     .option('--json', 'Output the claim in JSON format')
     // `options.wait` is default true. Passing `--no-wait` will set it to false.
     // https://github.com/tj/commander.js#other-option-types-negatable-boolean-and-booleanvalue
@@ -510,10 +516,28 @@ export function injectCommands(
         .default('60')
         .conflicts('wait'),
     )
-    .action(async (amount, recipient, options) => {
-      const { bridgeL1FeeJuice } = await import('./bridge_fee_juice.js');
-      const { l1ChainId, l1RpcUrls, l1PrivateKey, mnemonic, mint, json, wait, interval: intervalS } = options;
+    .action(async (amount, recipient, options, command) => {
+      const {
+        l1ChainId,
+        l1RpcUrls,
+        l1PrivateKey,
+        mnemonic,
+        mint,
+        json,
+        wait,
+        interval: intervalS,
+        iKnowThisUsesThePublicTestMnemonic,
+      } = options;
 
+      assertSafeL1Signer({
+        chainId: l1ChainId,
+        privateKey: l1PrivateKey,
+        mnemonic,
+        mnemonicWasExplicit: command.getOptionValueSource('mnemonic') !== 'default',
+        allowDefaultMnemonic: iKnowThisUsesThePublicTestMnemonic,
+      });
+
+      const { bridgeL1FeeJuice } = await import('./bridge_fee_juice.js');
       const [secret, messageLeafIndex] = await bridgeL1FeeJuice(
         amount,
         recipient,