diff --git a/extension/popup/screens/onboarding/ImportScreen.tsx b/extension/popup/screens/onboarding/ImportScreen.tsx
index 60b37b88..d20e6dfc 100644
--- a/extension/popup/screens/onboarding/ImportScreen.tsx
+++ b/extension/popup/screens/onboarding/ImportScreen.tsx
@@ -14,7 +14,11 @@ import { EyeIcon } from '../../components/icons/EyeIcon';
 import { EyeOffIcon } from '../../components/icons/EyeOffIcon';
 import { InfoIcon } from '../../components/icons/InfoIcon';
 import { importKeyfile, type Keyfile } from '../../../shared/keyfile';
-import { validateMnemonic } from '../../../shared/wallet-crypto';
+import { normalizeAndValidateMnemonic } from '../../../shared/wallet-crypto';
+
+function lowercaseAscii(value: string): string {
+  return value.replace(/[A-Z]/g, character => character.toLowerCase());
+}
 
 export function ImportScreen() {
   const {
@@ -51,7 +55,7 @@ export function ImportScreen() {
   const fileInputRef = useRef<HTMLInputElement>(null);
 
   function handleWordChange(index: number, value: string) {
-    const trimmedValue = value.trim().toLowerCase();
+    const trimmedValue = lowercaseAscii(value.trim());
     const newWords = [...words];
     newWords[index] = trimmedValue;
     setWords(newWords);
@@ -70,7 +74,7 @@ export function ImportScreen() {
   function handlePaste(index: number, e: React.ClipboardEvent<HTMLInputElement>) {
     if (index === 0) {
       const pasteData = e.clipboardData.getData('text');
-      const pastedWords = pasteData.trim().toLowerCase().split(/\s+/);
+      const pastedWords = lowercaseAscii(pasteData.trim()).split(/\s+/);
 
       if (pastedWords.length === UI_CONSTANTS.MNEMONIC_WORD_COUNT) {
         e.preventDefault();
@@ -117,7 +121,7 @@ export function ImportScreen() {
 
   async function handleImport(mnemonicOverride?: string) {
     // Use stored mnemonic (set either by manual entry or keyfile import)
-    const mnemonic = mnemonicOverride || onboardingMnemonic || words.join(' ').trim();
+    const mnemonicCandidate = mnemonicOverride || onboardingMnemonic || words.join(' ').trim();
 
     if (!isAddSeedFlow) {
       // Validate password
@@ -137,11 +141,14 @@ export function ImportScreen() {
       }
     }
 
+    const mnemonic = normalizeAndValidateMnemonic(mnemonicCandidate);
+    if (!mnemonic) {
+      setIsSubmitting(false);
+      setError('Invalid secret phrase. Please check your words and try again.');
+      return;
+    }
+
     if (isAddSeedFlow) {
-      if (!validateMnemonic(mnemonic)) {
-        setError('Invalid secret phrase. Please check your words and try again.');
-        return;
-      }
       setIsSubmitting(true);
       try {
         const result = await createMnemonicSeedSource(mnemonic, undefined, true);
diff --git a/extension/shared/vault.ts b/extension/shared/vault.ts
index 4cce04c0..cd6345a1 100644
--- a/extension/shared/vault.ts
+++ b/extension/shared/vault.ts
@@ -8,7 +8,7 @@ import {
   generateMnemonic,
   deriveAddress,
   deriveAddressFromMaster,
-  validateMnemonic,
+  normalizeAndValidateMnemonic,
 } from './wallet-crypto';
 import {
   ERROR_CODES,
@@ -531,8 +531,9 @@ export class Vault {
     mnemonic?: string
   ): Promise<{ ok: boolean; address: string; mnemonic: string } | { error: string }> {
     // Generate or validate mnemonic
-    const words = mnemonic ? mnemonic.trim() : generateMnemonic();
-    if (mnemonic && !validateMnemonic(words)) {
+    const mnemonicCandidate = mnemonic ? mnemonic : generateMnemonic();
+    const words = normalizeAndValidateMnemonic(mnemonicCandidate);
+    if (!words) {
       return { error: ERROR_CODES.INVALID_MNEMONIC };
     }
 
@@ -1050,8 +1051,9 @@ export class Vault {
       return { error: ERROR_CODES.LOCKED };
     }
 
-    const words = mnemonic ? mnemonic.trim() : generateMnemonic();
-    if (mnemonic && !validateMnemonic(words)) {
+    const mnemonicCandidate = mnemonic ? mnemonic : generateMnemonic();
+    const words = normalizeAndValidateMnemonic(mnemonicCandidate);
+    if (!words) {
       return { error: ERROR_CODES.INVALID_MNEMONIC };
     }
 
diff --git a/extension/shared/wallet-crypto.ts b/extension/shared/wallet-crypto.ts
index 8f39f353..f32b6d83 100644
--- a/extension/shared/wallet-crypto.ts
+++ b/extension/shared/wallet-crypto.ts
@@ -12,6 +12,8 @@ import wasm from './sdk-wasm.js';
 import { publicKeyToPKH } from './address-encoding';
 import { ensureWasmInitialized as ensureWasmInit } from './wasm-utils';
 
+const englishWords = new Set(wordlist);
+
 /**
  * Generates a BIP-39 mnemonic (24 words)
  * Uses 256 bits of entropy for maximum security
@@ -29,6 +31,39 @@ export function validateMnemonic(mnemonic: string): boolean {
   return validateMnemonicScure(mnemonic, wordlist);
 }
 
+export function normalizeAndValidateMnemonic(mnemonic: string): string | null {
+  const trimmed = mnemonic.trim();
+  if (!trimmed) return null;
+
+  const canonicalWords: string[] = [];
+  for (const rawToken of trimmed.split(/\s+/)) {
+    if (!/^[A-Za-z]+$/.test(rawToken)) return null;
+
+    const token = rawToken.toLowerCase();
+
+    // Exact words take precedence over four-character prefix expansion.
+    if (englishWords.has(token)) {
+      canonicalWords.push(token);
+      continue;
+    }
+
+    if (token.length !== 4) return null;
+
+    let prefixMatch: string | null = null;
+    for (const word of wordlist) {
+      if (!word.startsWith(token)) continue;
+      if (prefixMatch !== null) return null;
+      prefixMatch = word;
+    }
+
+    if (prefixMatch === null) return null;
+    canonicalWords.push(prefixMatch);
+  }
+
+  const canonicalMnemonic = canonicalWords.join(' ');
+  return validateMnemonic(canonicalMnemonic) ? canonicalMnemonic : null;
+}
+
 /**
  * Derives a Nockchain v1 PKH address from the master key (no child derivation)
  * This matches the CLI wallet behavior