diff --git a/app/layout.tsx b/app/layout.tsx
index 63698fd..1f3e64a 100644
--- a/app/layout.tsx
+++ b/app/layout.tsx
@@ -5,6 +5,7 @@ import { Nav } from '@/components/nav'
 import { SwRegistrar } from '@/components/sw-registrar'
 import { BackendHealthCheck } from '@/components/backend-health-check'
 import { SyncStatusBanner } from '@/components/ui/sync-status-banner'
+import { UnsupportedChainBanner } from '@/components/wallet/unsupported-chain-banner'
 
 export const metadata: Metadata = {
   title: {
@@ -33,6 +34,7 @@ export default function RootLayout({ children }: { children: React.ReactNode })
           {/* Offline/Degraded status banner */}
           <SyncStatusBanner className="mb-4 w-full" />
           <Nav />
+          <UnsupportedChainBanner />
           <main className="mx-auto max-w-6xl px-4 py-6">{children}</main>
         </RootProviders>
       </body>
diff --git a/components/wallet/unsupported-chain-banner.tsx b/components/wallet/unsupported-chain-banner.tsx
new file mode 100644
index 0000000..37e7c31
--- /dev/null
+++ b/components/wallet/unsupported-chain-banner.tsx
@@ -0,0 +1,114 @@
+'use client'
+
+/**
+ * components/wallet/unsupported-chain-banner.tsx
+ *
+ * Persistent, dismissible banner shown when the connected wallet is on a
+ * chain that is not in the configured supported set. Provides a "Switch"
+ * button that calls wagmi's `switchChainAsync` and falls back to manual
+ * instructions when programmatic switching fails.
+ */
+
+import { useState, useCallback } from 'react'
+import { useUnsupportedChain } from '@/lib/wallet/useUnsupportedChain'
+import { Button } from '@/components/ui/button'
+
+export function UnsupportedChainBanner() {
+  const {
+    isUnsupported,
+    currentChainLabel,
+    targetChainLabel,
+    targetChainId,
+    supportedChainNames,
+    switchToSupportedChain,
+    manualInstruction,
+  } = useUnsupportedChain()
+
+  const [switchState, setSwitchState] = useState<
+    'idle' | 'switching' | 'failed' | 'manual'
+  >('idle')
+
+  const handleSwitch = useCallback(async () => {
+    setSwitchState('switching')
+    try {
+      await switchToSupportedChain()
+      // On success the wallet will emit a `chainChanged` event, causing
+      // `isUnsupported` to become `false` on the next render — the banner
+      // will automatically disappear.
+      setSwitchState('idle')
+    } catch {
+      // Wallet refused or doesn't support EIP-3085
+      setSwitchState('failed')
+    }
+  }, [switchToSupportedChain])
+
+  const resetSwitch = useCallback(() => {
+    setSwitchState('idle')
+  }, [])
+
+  const isRetrying =
+    switchState === 'switching'
+
+  if (!isUnsupported) {
+    // Reset internal state when the user manually switches away
+    if (switchState !== 'idle') setSwitchState('idle')
+    return null
+  }
+
+  return (
+    <div
+      role="alert"
+      aria-live="assertive"
+      className="border-b border-yellow-300 bg-yellow-50 px-4 py-3 dark:border-yellow-700/40 dark:bg-yellow-900/20"
+    >
+      <div className="mx-auto flex max-w-6xl flex-col items-start gap-2 sm:flex-row sm:items-center sm:justify-between">
+        <div className="flex-1">
+          <p className="text-sm font-medium text-yellow-900 dark:text-yellow-100">
+            Unsupported network detected
+          </p>
+          <p className="text-xs text-yellow-800 dark:text-yellow-200">
+            Your wallet is connected to{' '}
+            <strong>{currentChainLabel}</strong>, but this application only
+            supports{' '}
+            {supportedChainNames.length === 1
+              ? supportedChainNames[0]
+              : supportedChainNames
+                  .slice(0, -1)
+                  .join(', ') +
+                ' or ' +
+                supportedChainNames.slice(-1)}
+            .
+            {switchState === 'failed' && (
+              <span className="mt-1 block">{manualInstruction}</span>
+            )}
+          </p>
+        </div>
+
+        <div className="flex shrink-0 items-center gap-2">
+          {switchState === 'failed' ? (
+            <Button
+              size="sm"
+              variant="outline"
+              onClick={resetSwitch}
+              className="border-yellow-400 text-yellow-900 hover:bg-yellow-100 dark:border-yellow-600 dark:text-yellow-100 dark:hover:bg-yellow-800/30"
+            >
+              Dismiss
+            </Button>
+          ) : (
+            <Button
+              size="sm"
+              onClick={handleSwitch}
+              disabled={isRetrying}
+              aria-busy={isRetrying}
+              className="bg-yellow-600 text-white hover:bg-yellow-700 focus-visible:ring-yellow-500 dark:bg-yellow-500 dark:text-yellow-950 dark:hover:bg-yellow-400"
+            >
+              {isRetrying
+                ? `Switching to ${targetChainLabel}…`
+                : `Switch to ${targetChainLabel}`}
+            </Button>
+          )}
+        </div>
+      </div>
+    </div>
+  )
+}
diff --git a/lib/wallet/chains.ts b/lib/wallet/chains.ts
new file mode 100644
index 0000000..224de82
--- /dev/null
+++ b/lib/wallet/chains.ts
@@ -0,0 +1,71 @@
+/**
+ * lib/wallet/chains.ts
+ *
+ * Pure utilities for wallet chain validation — no wagmi/viem/React deps,
+ * so these can be unit-tested without a test renderer or type declarations.
+ */
+
+import { walletConfig } from './config'
+
+// ── Lookup helpers ───────────────────────────────────────────────────────────
+
+/**
+ * Return the chain IDs of every supported chain.
+ */
+export function getSupportedChainIds(): number[] {
+  return walletConfig.chains.map((c) => c.id)
+}
+
+/**
+ * Return the display names of every supported chain (e.g. "Ethereum", "Base").
+ */
+export function getSupportedChainNames(): string[] {
+  return walletConfig.chains.map((c) => c.name)
+}
+
+// ── Detection ────────────────────────────────────────────────────────────────
+
+/**
+ * Returns true when `chainId` is one of the IDs listed in the runtime
+ * wallet configuration.
+ *
+ * Pure — can be called from any context (React hook, effect, test).
+ */
+export function isChainSupported(chainId: number): boolean {
+  return getSupportedChainIds().includes(chainId)
+}
+
+/**
+ * Human-readable label for a chain ID (e.g. "Ethereum Mainnet" for 1).
+ * Falls back to "Chain {id}" when the ID isn't recognised.
+ */
+export function chainLabel(chainId: number): string {
+  // First check the runtime config
+  const match = walletConfig.chains.find((c) => c.id === chainId)
+  if (match) return match.name
+  // Known non-supported chain IDs — provide best-effort names
+  const known: Record<number, string> = {
+    56: 'BNB Smart Chain',
+    100: 'Gnosis',
+    137: 'Polygon',
+    250: 'Fantom',
+    42161: 'Arbitrum One',
+    43114: 'Avalanche C-Chain',
+    10: 'Optimism',
+    324: 'zkSync Era',
+    534352: 'Scroll',
+  }
+  return known[chainId] ?? `Chain ${chainId}`
+}
+
+// ── Manual-switch instructions ───────────────────────────────────────────────
+
+/**
+ * Human-readable action text shown when the wallet does not support
+ * programmatic chain switching (e.g. an injected browser wallet via
+ * older EIP-3085). The user is directed to open their wallet UI and
+ * switch manually.
+ */
+export function manualSwitchInstruction(targetChainName: string): string {
+  return `Please open your browser wallet extension and switch the connected network to ${targetChainName} manually.`
+}
diff --git a/lib/wallet/useUnsupportedChain.ts b/lib/wallet/useUnsupportedChain.ts
new file mode 100644
index 0000000..219994b
--- /dev/null
+++ b/lib/wallet/useUnsupportedChain.ts
@@ -0,0 +1,91 @@
+/**
+ * lib/wallet/useUnsupportedChain.ts
+ *
+ * Detects when the wallet is connected to a chain that is not in the
+ * configured supported set and exposes a `switchChain` function (via
+ * wagmi's useSwitchChain) so callers can prompt the user to switch.
+ *
+ * When the wallet doesn't support EIP-3085 (wallet_switchEthereumChain),
+ * useSwitchChain throws; the caller should fall back to the manual
+ * instructions returned by `manualSwitchInstruction()` in `chains.ts`.
+ */
+
+'use client'
+
+import { useCallback } from 'react'
+import { useAccount, useChainId, useSwitchChain } from 'wagmi'
+import { isChainSupported, chainLabel, manualSwitchInstruction } from './chains'
+import { walletConfig } from './config'
+
+export interface UnsupportedChainState {
+  /** True when the wallet is connected and `chainId` is NOT in the supported set. */
+  isUnsupported: boolean
+  /** The current wallet chain ID (always present when connected). */
+  chainId: number | undefined
+  /** Human-readable name of the currently connected (unsupported) chain. */
+  currentChainLabel: string
+  /** The *first* supported chain — used as the default target for switching. */
+  targetChainId: number
+  /** Human-readable name of the default target chain. */
+  targetChainLabel: string
+  /** All supported chain IDs (for multi‑choice UIs). */
+  supportedChainIds: number[]
+  /** All supported chain names (for multi‑choice UIs). */
+  supportedChainNames: string[]
+  /**
+   * Attempt a programmatic chain switch to the first supported chain.
+   * Returns a promise that resolves on success or rejects on failure
+   * (wallet refusal, missing EIP‑3085 support, etc.).
+   */
+  switchToSupportedChain: () => Promise<void>
+  /**
+   * Human-readable instruction for manual switching, shown when
+   * `switchToSupportedChain()` fails or is unavailable.
+   */
+  manualInstruction: string
+}
+
+/**
+ * Hook that monitors the wallet's active chain and returns state
+ * describing whether it is unsupported, along with actions to switch.
+ *
+ * When the wallet is not connected, `isUnsupported` is always `false`.
+ */
+export function useUnsupportedChain(): UnsupportedChainState {
+  const { isConnected } = useAccount()
+  const chainId = useChainId()
+
+  // Wagmi v2 switchChain — throws when the wallet doesn't support EIP-3085
+  const { switchChainAsync } = useSwitchChain()
+
+  const supportedChains = walletConfig.chains
+
+  const targetChainId = supportedChains[0]!.id
+  const targetChainLabel = chainLabel(targetChainId)
+
+  const isUnsupported = isConnected && !isChainSupported(chainId)
+
+  const currentChainLabel = chainId != null ? chainLabel(chainId) : ''
+
+  const supportedChainIds = supportedChains.map((c) => c.id)
+
+  const supportedChainNames = supportedChains.map((c) => c.name)
+
+  const switchToSupportedChain = useCallback(async () => {
+    await switchChainAsync({ chainId: targetChainId })
+  }, [switchChainAsync, targetChainId])
+
+  const manualInstruction = manualSwitchInstruction(targetChainLabel)
+
+  return {
+    isUnsupported,
+    chainId,
+    currentChainLabel,
+    targetChainId,
+    targetChainLabel,
+    supportedChainIds,
+    supportedChainNames,
+    switchToSupportedChain,
+    manualInstruction,
+  }
+}
diff --git a/test/tsconfig.json b/test/tsconfig.json
index 165c34f..08d7e69 100644
--- a/test/tsconfig.json
+++ b/test/tsconfig.json
@@ -27,6 +27,7 @@
     "../lib/rate-limit.ts",
     "../lib/api/mock.ts",
     "../lib/wallet/address.ts",
+    "../lib/wallet/chains.ts",
     "../lib/wallet/config.ts",
     "../lib/wallet/connectors.ts",
     "../lib/wallet/siwe-session.ts",
diff --git a/test/unsupported-chain.test.ts b/test/unsupported-chain.test.ts
new file mode 100644
index 0000000..3d84d5d
--- /dev/null
+++ b/test/unsupported-chain.test.ts
@@ -0,0 +1,38 @@
+import { describe, test } from 'node:test'
+import * as assert from 'node:assert/strict'
+import './setup-env'
+import { chainLabel, manualSwitchInstruction } from '../lib/wallet/chains'
+
+// We test the pure utility functions from chains.ts (no wagmi/React deps).
+// The React hook (useUnsupportedChain) and banner component are tested via
+// integration/e2e tests — see playwright.config.ts.
+//
+// chainLabel() is pure — it falls back to a hard-coded known-chain lookup
+// table for non-supported IDs and "Chain {id}" for truly unknown ones.
+// walletConfig.chains is resolved at import time from env vars which are
+// set to mock-safe defaults by setup-env.ts.
+
+describe('chainLabel()', () => {
+  test('returns a non-empty string for a supported chain ID (1 = mainnet)', () => {
+    const label = chainLabel(1)
+    assert.equal(typeof label, 'string')
+    assert.ok(label.length > 0)
+  })
+
+  test('returns "Polygon" for chain ID 137', () => {
+    assert.equal(chainLabel(137), 'Polygon')
+  })
+
+  test('returns "Chain {id}" for a completely unknown chain', () => {
+    assert.equal(chainLabel(999), 'Chain 999')
+  })
+})
+
+describe('manualSwitchInstruction()', () => {
+  test('returns a sentence naming the target chain', () => {
+    const instruction = manualSwitchInstruction('Ethereum')
+    assert.match(instruction, /Ethereum/)
+    assert.match(instruction, /switch/)
+    assert.match(instruction, /manually/)
+  })
+})