diff --git a/client/src/App.tsx b/client/src/App.tsx
index 06a6ec4..3896413 100644
--- a/client/src/App.tsx
+++ b/client/src/App.tsx
@@ -1,6 +1,7 @@
-import { useState } from 'react'
+import { useState, useEffect } from 'react'
 import { useAuth } from './hooks/useAuth'
 import { LoginPage } from './pages/LoginPage'
+import { OnboardingPage } from './pages/OnboardingPage'
 import './index.css'
 
 type AppRoute = 'login' | 'sent' | 'onboarding' | 'game' | 'admin'
@@ -8,8 +9,27 @@ type AppRoute = 'login' | 'sent' | 'onboarding' | 'game' | 'admin'
 export default function App() {
   const { profile, token, loading, logout } = useAuth()
   const [route, setRoute] = useState<AppRoute>('login')
+  const [checkingOnboarding, setCheckingOnboarding] = useState(false)
 
-  if (loading) {
+  // After auth resolves, check onboarding status
+  useEffect(() => {
+    if (!profile || !token) return
+    if (route !== 'login' && route !== 'sent') return
+
+    setCheckingOnboarding(true)
+    fetch('/api/mnemonics', {
+      headers: { Authorization: `Bearer ${token}` },
+    })
+      .then(r => r.json())
+      .then((mnemonics: Record<string, string>) => {
+        const count = Object.keys(mnemonics).length
+        setRoute(count >= 12 ? 'game' : 'onboarding')
+      })
+      .catch(() => setRoute('game'))
+      .finally(() => setCheckingOnboarding(false))
+  }, [profile, token])
+
+  if (loading || checkingOnboarding) {
     return (
       <div className="app-shell" style={{ textAlign: 'center', paddingTop: '4rem' }}>
         <p style={{ fontFamily: 'var(--font-header)' }}>Loading quest...</p>
@@ -17,28 +37,21 @@ export default function App() {
     )
   }
 
-  // If authenticated and still on an unauthenticated route, go to game
-  // (Task 10 will refine this to check onboarding completion)
-  const activeRoute: AppRoute = profile && (route === 'login' || route === 'sent') ? 'game' : route
-
   return (
     <div className="app-shell">
-      {activeRoute === 'login' && (
+      {route === 'login' && (
         <LoginPage onLoginSent={() => setRoute('sent')} />
       )}
-      {activeRoute === 'sent' && (
+      {route === 'sent' && (
         <div style={{ maxWidth: 420, margin: '4rem auto 0', textAlign: 'center' }}>
           <h2 style={{ fontFamily: 'var(--font-header)', marginBottom: '1rem' }}>Check your email</h2>
           <p>We sent a magic link to your inbox. Click it to begin your quest!</p>
         </div>
       )}
-      {activeRoute === 'onboarding' && (
-        <div>
-          <h1 style={{ fontFamily: 'var(--font-header)' }}>Welcome, {profile?.display_name || 'Adventurer'}!</h1>
-          <p>Mnemonic builder coming in Task 10.</p>
-        </div>
+      {route === 'onboarding' && token && (
+        <OnboardingPage token={token} onComplete={() => setRoute('game')} />
       )}
-      {activeRoute === 'game' && (
+      {route === 'game' && (
         <div>
           <h1 style={{ fontFamily: 'var(--font-header)' }}>Practice</h1>
           <p>Hello, {profile?.display_name || profile?.email || 'Adventurer'}!</p>
@@ -48,7 +61,7 @@ export default function App() {
           </button>
         </div>
       )}
-      {activeRoute === 'admin' && (
+      {route === 'admin' && (
         <div>
           <h1 style={{ fontFamily: 'var(--font-header)' }}>Admin</h1>
           <p>Admin panel coming in Task 12.</p>
diff --git a/client/src/components/MnemonicBuilder.tsx b/client/src/components/MnemonicBuilder.tsx
new file mode 100644
index 0000000..cc31eff
--- /dev/null
+++ b/client/src/components/MnemonicBuilder.tsx
@@ -0,0 +1,143 @@
+import { useState } from 'react'
+import { NOVICE_LETTERS, MORSE, validatePhrase } from '../lib/morse'
+
+type Props = {
+  token: string
+  onComplete: () => void
+}
+
+const LETTERS = NOVICE_LETTERS  // ['A','E','T','I','N','S','H','R','D','L','U','O']
+
+export function MnemonicBuilder({ token, onComplete }: Props) {
+  const [index, setIndex] = useState(0)
+  const [phrase, setPhrase] = useState('')
+  const [saving, setSaving] = useState(false)
+  const [error, setError] = useState<string | null>(null)
+
+  const letter = LETTERS[index]
+  const pattern = MORSE[letter] ?? ''
+  // Display pattern as dots/dashes: '.' → '·', '-' → '—'
+  const display = pattern.split('').map(c => c === '.' ? '·' : '—').join(' ')
+  const validation = phrase.trim() ? validatePhrase(phrase.trim(), letter) : null
+
+  async function save() {
+    if (!validation?.valid) return
+    setSaving(true)
+    setError(null)
+    try {
+      const r = await fetch('/api/mnemonics', {
+        method: 'POST',
+        headers: {
+          'Content-Type': 'application/json',
+          Authorization: `Bearer ${token}`,
+        },
+        body: JSON.stringify({ letter, phrase: phrase.trim() }),
+      })
+      if (!r.ok) throw new Error('Failed to save')
+      advance()
+    } catch {
+      setError('Failed to save. Try again.')
+    } finally {
+      setSaving(false)
+    }
+  }
+
+  function advance() {
+    if (index + 1 >= LETTERS.length) {
+      onComplete()
+    } else {
+      setIndex(i => i + 1)
+      setPhrase('')
+      setError(null)
+    }
+  }
+
+  return (
+    <div style={{ maxWidth: 520, margin: '0 auto' }}>
+      {/* Progress */}
+      <p style={{ color: 'var(--color-brown)', marginBottom: '1.5rem' }}>
+        Letter {index + 1} of {LETTERS.length}
+      </p>
+
+      {/* Letter display */}
+      <div style={{ textAlign: 'center', marginBottom: '2rem' }}>
+        <div style={{
+          fontSize: '4rem',
+          fontFamily: 'var(--font-header)',
+          color: 'var(--color-forest)',
+          lineHeight: 1,
+        }}>
+          {letter}
+        </div>
+        <div style={{
+          fontSize: '1.8rem',
+          fontFamily: 'var(--font-mono)',
+          color: 'var(--color-gold)',
+          marginTop: '0.5rem',
+          letterSpacing: '0.3em',
+        }}>
+          {display}
+        </div>
+        <div style={{ fontSize: '0.85rem', color: 'var(--color-brown)', marginTop: '0.25rem' }}>
+          {pattern.length} {pattern.length === 1 ? 'word' : 'words'} needed
+        </div>
+      </div>
+
+      {/* Phrase input */}
+      <label style={{ display: 'block', fontWeight: 600, marginBottom: '0.4rem' }}>
+        Create your mnemonic phrase:
+      </label>
+      <p style={{ fontSize: '0.85rem', color: 'var(--color-brown)', marginBottom: '0.5rem' }}>
+        {pattern.length} word{pattern.length !== 1 ? 's' : ''} — short words (1-2 letters) for dots,
+        long words (3+ letters) for dashes.
+      </p>
+      <input
+        type="text"
+        value={phrase}
+        onChange={e => { setPhrase(e.target.value); setError(null) }}
+        placeholder={`${pattern.length} word${pattern.length !== 1 ? 's' : ''}, e.g. "${examplePhrase(pattern)}"`}
+        disabled={saving}
+        style={{ marginBottom: '0.5rem' }}
+      />
+
+      {/* Validation feedback */}
+      {validation && !validation.valid && (
+        <p style={{ color: 'crimson', fontSize: '0.9rem', marginBottom: '0.5rem' }}>
+          {validation.message}
+        </p>
+      )}
+      {validation?.valid && (
+        <p style={{ color: 'green', fontSize: '0.9rem', marginBottom: '0.5rem' }}>
+          Looks good!
+        </p>
+      )}
+      {error && (
+        <p style={{ color: 'crimson', fontSize: '0.9rem', marginBottom: '0.5rem' }}>{error}</p>
+      )}
+
+      {/* Actions */}
+      <div style={{ display: 'flex', gap: '0.75rem', marginTop: '1rem' }}>
+        <button
+          className="btn"
+          onClick={save}
+          disabled={saving || !validation?.valid}
+          style={{ flex: 1 }}
+        >
+          {saving ? 'Saving...' : 'Save (+50 pts)'}
+        </button>
+        <button
+          className="btn btn-ghost"
+          onClick={advance}
+          disabled={saving}
+        >
+          Skip
+        </button>
+      </div>
+    </div>
+  )
+}
+
+// Generate a simple example phrase for the placeholder
+function examplePhrase(pattern: string): string {
+  return pattern.split('').map(c => c === '.' ? 'go' : 'WALKING').join(' ')
+}
diff --git a/client/src/pages/OnboardingPage.tsx b/client/src/pages/OnboardingPage.tsx
new file mode 100644
index 0000000..5bf4357
--- /dev/null
+++ b/client/src/pages/OnboardingPage.tsx
@@ -0,0 +1,20 @@
+import { MnemonicBuilder } from '../components/MnemonicBuilder'
+
+type Props = {
+  token: string
+  onComplete: () => void
+}
+
+export function OnboardingPage({ token, onComplete }: Props) {
+  return (
+    <div>
+      <h1 style={{ fontFamily: 'var(--font-header)', marginBottom: '0.5rem' }}>
+        Build Your Mnemonics
+      </h1>
+      <p style={{ color: 'var(--color-brown)', marginBottom: '2rem' }}>
+        Create a memorable phrase for each Morse letter. Short words for dots, long words for dashes.
+      </p>
+      <MnemonicBuilder token={token} onComplete={onComplete} />
+    </div>
+  )
+}