diff --git a/packages/keyring-api/CHANGELOG.md b/packages/keyring-api/CHANGELOG.md
index 87d6da0bd..16b9aa20d 100644
--- a/packages/keyring-api/CHANGELOG.md
+++ b/packages/keyring-api/CHANGELOG.md
@@ -7,6 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Add `{Balance,FungibleAssetAmountStruct}.metadata` ([#601](https://github.com/MetaMask/accounts/pull/601))
+  - This can be used to pass extra-metadata alongside assets balances.
+  - `FungibleAssetAmountStruct` now composes `BalanceStruct`
+  - `notify:accountBalancesUpdated` also accepts `metadata` now.
+
 ## [23.6.0]
 
 ### Added
diff --git a/packages/keyring-api/src/api/asset.test-d.ts b/packages/keyring-api/src/api/asset.test-d.ts
index f150b7210..bd162d502 100644
--- a/packages/keyring-api/src/api/asset.test-d.ts
+++ b/packages/keyring-api/src/api/asset.test-d.ts
@@ -9,6 +9,30 @@ expectAssignable<Asset>({
   amount: '0.01',
 });
 
+expectAssignable<Asset>({
+  fungible: true,
+  type: 'eip155:1/slip44:60',
+  unit: 'ETH',
+  amount: '0.01',
+  metadata: { foo: 'bar', count: 42, flag: true },
+});
+
+expectNotAssignable<Asset>({
+  fungible: true,
+  type: 'eip155:1/slip44:60',
+  unit: 'ETH',
+  amount: '0.01',
+  metadata: 'string',
+});
+
+expectNotAssignable<Asset>({
+  fungible: true,
+  type: 'eip155:1/slip44:60',
+  unit: 'ETH',
+  amount: '0.01',
+  metadata: 42,
+});
+
 expectAssignable<Asset>({
   fungible: false,
   id: 'hedera:mainnet/nft:0.0.55492/12',
diff --git a/packages/keyring-api/src/api/asset.test.ts b/packages/keyring-api/src/api/asset.test.ts
index 572086554..861c6ead4 100644
--- a/packages/keyring-api/src/api/asset.test.ts
+++ b/packages/keyring-api/src/api/asset.test.ts
@@ -36,6 +36,28 @@ describe('AssetStruct', () => {
       },
       expected: true,
     },
+    // Valid with metadata
+    {
+      asset: {
+        fungible: true,
+        type: 'eip155:1/slip44:60',
+        unit: 'ETH',
+        amount: '0.01',
+        metadata: { foo: 'bar', count: 42 },
+      },
+      expected: true,
+    },
+    // Invalid metadata type
+    {
+      asset: {
+        fungible: true,
+        type: 'eip155:1/slip44:60',
+        unit: 'ETH',
+        amount: '0.01',
+        metadata: 'string',
+      },
+      expected: false,
+    },
   ])('returns $expected for is($asset, AssetStruct)', ({ asset, expected }) => {
     expect(is(asset, AssetStruct)).toBe(expected);
   });
diff --git a/packages/keyring-api/src/api/asset.ts b/packages/keyring-api/src/api/asset.ts
index 04be00d22..98314f55f 100644
--- a/packages/keyring-api/src/api/asset.ts
+++ b/packages/keyring-api/src/api/asset.ts
@@ -1,25 +1,19 @@
-import { selectiveUnion, StringNumberStruct } from '@metamask/keyring-utils';
+import { selectiveUnion } from '@metamask/keyring-utils';
 import type { Infer } from '@metamask/superstruct';
-import { literal, object, string } from '@metamask/superstruct';
+import { literal, object } from '@metamask/superstruct';
 import {
   CaipAssetIdStruct,
   CaipAssetTypeStruct,
   isPlainObject,
 } from '@metamask/utils';
 
+import { BalanceStruct } from './balance';
+
 /**
  * Fungible asset amount struct.
  */
 export const FungibleAssetAmountStruct = object({
-  /**
-   * Asset unit.
-   */
-  unit: string(),
-
-  /**
-   * Asset amount.
-   */
-  amount: StringNumberStruct,
+  ...BalanceStruct.schema,
 });
 
 /**
diff --git a/packages/keyring-api/src/api/balance.test-d.ts b/packages/keyring-api/src/api/balance.test-d.ts
index ccb0cc662..47f59e689 100644
--- a/packages/keyring-api/src/api/balance.test-d.ts
+++ b/packages/keyring-api/src/api/balance.test-d.ts
@@ -17,3 +17,28 @@ expectNotAssignable<Balance>({ amount: '1.0', unit: 1 });
 expectNotAssignable<Balance>({ amount: '1.0', unit: true });
 expectNotAssignable<Balance>({ amount: '1.0', unit: undefined });
 expectNotAssignable<Balance>({ amount: '1.0', unit: null });
+
+// metadata is optional and accepts arbitrary JSON-compatible key/value pairs
+expectAssignable<Balance>({
+  amount: '1.0',
+  unit: 'ETH',
+  metadata: { foo: 'bar' },
+});
+expectAssignable<Balance>({
+  amount: '1.0',
+  unit: 'ETH',
+  metadata: { count: 42, flag: true },
+});
+expectAssignable<Balance>({
+  amount: '1.0',
+  unit: 'ETH',
+  metadata: { nested: { x: 1 } },
+});
+expectAssignable<Balance>({ amount: '1.0', unit: 'ETH' });
+
+expectNotAssignable<Balance>({
+  amount: '1.0',
+  unit: 'ETH',
+  metadata: 'string',
+});
+expectNotAssignable<Balance>({ amount: '1.0', unit: 'ETH', metadata: 42 });
diff --git a/packages/keyring-api/src/api/balance.test.ts b/packages/keyring-api/src/api/balance.test.ts
index 27ce466fb..63b7f2f69 100644
--- a/packages/keyring-api/src/api/balance.test.ts
+++ b/packages/keyring-api/src/api/balance.test.ts
@@ -19,6 +19,34 @@ describe('BalanceStruct', () => {
     { balance: { amount: '1.0', unit: 1 }, expected: false },
     { balance: { amount: '1.0', unit: true }, expected: false },
     { balance: { amount: '1.0', unit: null }, expected: false },
+    // With metadata
+    {
+      balance: { amount: '1.0', unit: 'ETH', metadata: { foo: 'bar' } },
+      expected: true,
+    },
+    {
+      balance: {
+        amount: '1.0',
+        unit: 'ETH',
+        metadata: { count: 42, flag: true, nested: { x: 1 } },
+      },
+      expected: true,
+    },
+    // Without metadata (optional — key must be absent, not explicitly undefined)
+    {
+      balance: { amount: '1.0', unit: 'ETH', metadata: undefined },
+      expected: false,
+    },
+    // Invalid metadata values
+    {
+      balance: { amount: '1.0', unit: 'ETH', metadata: 'string' },
+      expected: false,
+    },
+    { balance: { amount: '1.0', unit: 'ETH', metadata: 42 }, expected: false },
+    {
+      balance: { amount: '1.0', unit: 'ETH', metadata: { key: undefined } },
+      expected: false,
+    },
   ])(
     'returns $expected for is($balance, BalanceStruct)',
     ({ balance, expected }) => {
diff --git a/packages/keyring-api/src/api/balance.ts b/packages/keyring-api/src/api/balance.ts
index 2ab2a2989..02231a928 100644
--- a/packages/keyring-api/src/api/balance.ts
+++ b/packages/keyring-api/src/api/balance.ts
@@ -1,10 +1,23 @@
 import { StringNumberStruct } from '@metamask/keyring-utils';
 import type { Infer } from '@metamask/superstruct';
-import { object, string } from '@metamask/superstruct';
+import { exactOptional, object, record, string } from '@metamask/superstruct';
+import { JsonStruct } from '@metamask/utils';
 
 export const BalanceStruct = object({
+  /**
+   * Asset amount.
+   */
   amount: StringNumberStruct,
+
+  /**
+   * Asset unit.
+   */
   unit: string(),
+
+  /**
+   * Optional arbitrary metadata associated with this balance.
+   */
+  metadata: exactOptional(record(string(), JsonStruct)),
 });
 
 export type Balance = Infer<typeof BalanceStruct>;