diff --git a/.changeset/tx-envelope-chain-id-assert.md b/.changeset/tx-envelope-chain-id-assert.md
new file mode 100644
index 00000000..babf9a7e
--- /dev/null
+++ b/.changeset/tx-envelope-chain-id-assert.md
@@ -0,0 +1,5 @@
+---
+"ox": patch
+---
+
+Fixed `TxEnvelopeLegacy.assert`, `TxEnvelopeEip2930.assert`, `TxEnvelopeEip1559.assert` and `TxEnvelopeTempo.assert` to throw `InvalidChainIdError` for a `chainId` that is not a positive safe integer.
diff --git a/src/core/TxEnvelopeEip1559.ts b/src/core/TxEnvelopeEip1559.ts
index 35ed409b..fb8db935 100644
--- a/src/core/TxEnvelopeEip1559.ts
+++ b/src/core/TxEnvelopeEip1559.ts
@@ -71,7 +71,10 @@ export type Type = typeof type
  */
 export function assert(envelope: PartialBy<TxEnvelopeEip1559, 'type'>) {
   const { chainId, maxPriorityFeePerGas, maxFeePerGas, to } = envelope
-  if (chainId <= 0)
+  if (
+    typeof chainId !== 'undefined' &&
+    (!Number.isSafeInteger(chainId) || chainId <= 0)
+  )
     throw new TransactionEnvelope.InvalidChainIdError({ chainId })
   if (to) Address.assert(to, { strict: false })
   if (maxFeePerGas && BigInt(maxFeePerGas) > 2n ** 256n - 1n)
diff --git a/src/core/TxEnvelopeEip2930.ts b/src/core/TxEnvelopeEip2930.ts
index ddbf6169..ebc67979 100644
--- a/src/core/TxEnvelopeEip2930.ts
+++ b/src/core/TxEnvelopeEip2930.ts
@@ -69,7 +69,10 @@ export type Type = typeof type
  */
 export function assert(envelope: PartialBy<TxEnvelopeEip2930, 'type'>) {
   const { chainId, gasPrice, to } = envelope
-  if (chainId <= 0)
+  if (
+    typeof chainId !== 'undefined' &&
+    (!Number.isSafeInteger(chainId) || chainId <= 0)
+  )
     throw new TransactionEnvelope.InvalidChainIdError({ chainId })
   if (to) Address.assert(to, { strict: false })
   if (gasPrice && BigInt(gasPrice) > 2n ** 256n - 1n)
diff --git a/src/core/TxEnvelopeLegacy.ts b/src/core/TxEnvelopeLegacy.ts
index 3f8a5f05..036dc1dc 100644
--- a/src/core/TxEnvelopeLegacy.ts
+++ b/src/core/TxEnvelopeLegacy.ts
@@ -68,7 +68,10 @@ export type Type = typeof type
 export function assert(envelope: PartialBy<TxEnvelopeLegacy, 'type'>) {
   const { chainId, gasPrice, to } = envelope
   if (to) Address.assert(to, { strict: false })
-  if (typeof chainId !== 'undefined' && chainId <= 0)
+  if (
+    typeof chainId !== 'undefined' &&
+    (!Number.isSafeInteger(chainId) || chainId <= 0)
+  )
     throw new TransactionEnvelope.InvalidChainIdError({ chainId })
   if (gasPrice && BigInt(gasPrice) > 2n ** 256n - 1n)
     throw new TransactionEnvelope.GasPriceTooHighError({ gasPrice })
diff --git a/src/core/_test/TransactionEnvelopeEip1559.test.ts b/src/core/_test/TransactionEnvelopeEip1559.test.ts
index 250f7593..837d9c41 100644
--- a/src/core/_test/TransactionEnvelopeEip1559.test.ts
+++ b/src/core/_test/TransactionEnvelopeEip1559.test.ts
@@ -47,6 +47,34 @@ describe('assert', () => {
     )
   })
 
+  test('invalid chainId (not a positive safe integer)', () => {
+    expect(() =>
+      TxEnvelopeEip1559.assert({ chainId: Number.NaN }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "NaN" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeEip1559.assert({ chainId: 1.5 }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "1.5" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeEip1559.assert({ chainId: Number.MAX_SAFE_INTEGER + 1 }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "9007199254740992" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeEip1559.assert({ chainId: '1' as never }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "1" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeEip1559.assert({ chainId: 1n as never }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "1" is invalid.]`,
+    )
+  })
+
   test('invalid address', () => {
     expect(() => TxEnvelopeEip1559.assert({ to: '0x123', chainId: 1 }))
       .toThrowErrorMatchingInlineSnapshot(`
diff --git a/src/core/_test/TransactionEnvelopeEip2930.test.ts b/src/core/_test/TransactionEnvelopeEip2930.test.ts
index 15baa6ad..5513c19f 100644
--- a/src/core/_test/TransactionEnvelopeEip2930.test.ts
+++ b/src/core/_test/TransactionEnvelopeEip2930.test.ts
@@ -23,6 +23,34 @@ describe('assert', () => {
     )
   })
 
+  test('invalid chainId (not a positive safe integer)', () => {
+    expect(() =>
+      TxEnvelopeEip2930.assert({ chainId: Number.NaN }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "NaN" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeEip2930.assert({ chainId: 1.5 }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "1.5" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeEip2930.assert({ chainId: Number.MAX_SAFE_INTEGER + 1 }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "9007199254740992" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeEip2930.assert({ chainId: '1' as never }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "1" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeEip2930.assert({ chainId: 1n as never }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "1" is invalid.]`,
+    )
+  })
+
   test('invalid address', () => {
     expect(() => TxEnvelopeEip2930.assert({ to: '0x123', chainId: 1 }))
       .toThrowErrorMatchingInlineSnapshot(`
diff --git a/src/core/_test/TransactionEnvelopeLegacy.test.ts b/src/core/_test/TransactionEnvelopeLegacy.test.ts
index adff927f..6412dea8 100644
--- a/src/core/_test/TransactionEnvelopeLegacy.test.ts
+++ b/src/core/_test/TransactionEnvelopeLegacy.test.ts
@@ -23,6 +23,34 @@ describe('assert', () => {
     )
   })
 
+  test('invalid chainId (not a positive safe integer)', () => {
+    expect(() =>
+      TxEnvelopeLegacy.assert({ chainId: Number.NaN }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "NaN" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeLegacy.assert({ chainId: 1.5 }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "1.5" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeLegacy.assert({ chainId: Number.MAX_SAFE_INTEGER + 1 }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "9007199254740992" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeLegacy.assert({ chainId: '1' as never }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "1" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeLegacy.assert({ chainId: 1n as never }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "1" is invalid.]`,
+    )
+  })
+
   test('invalid address', () => {
     expect(() => TxEnvelopeLegacy.assert({ to: '0x123', chainId: 1 }))
       .toThrowErrorMatchingInlineSnapshot(`
diff --git a/src/tempo/TxEnvelopeTempo.test.ts b/src/tempo/TxEnvelopeTempo.test.ts
index 3c21cfd5..a3f34c0e 100644
--- a/src/tempo/TxEnvelopeTempo.test.ts
+++ b/src/tempo/TxEnvelopeTempo.test.ts
@@ -174,6 +174,49 @@ describe('assert', () => {
     )
   })
 
+  test('invalid chain id (not a positive safe integer)', () => {
+    expect(() =>
+      TxEnvelopeTempo.assert({
+        calls: [{ to: '0x0000000000000000000000000000000000000000' }],
+        chainId: Number.NaN,
+      }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "NaN" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeTempo.assert({
+        calls: [{ to: '0x0000000000000000000000000000000000000000' }],
+        chainId: 1.5,
+      }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "1.5" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeTempo.assert({
+        calls: [{ to: '0x0000000000000000000000000000000000000000' }],
+        chainId: Number.MAX_SAFE_INTEGER + 1,
+      }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "9007199254740992" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeTempo.assert({
+        calls: [{ to: '0x0000000000000000000000000000000000000000' }],
+        chainId: '1' as never,
+      }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "1" is invalid.]`,
+    )
+    expect(() =>
+      TxEnvelopeTempo.assert({
+        calls: [{ to: '0x0000000000000000000000000000000000000000' }],
+        chainId: 1n as never,
+      }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `[TransactionEnvelope.InvalidChainIdError: Chain ID "1" is invalid.]`,
+    )
+  })
+
   test('invalid fee payer signature', () => {
     const envelope = {
       calls: [{ to: '0x0000000000000000000000000000000000000000' }],
diff --git a/src/tempo/TxEnvelopeTempo.ts b/src/tempo/TxEnvelopeTempo.ts
index ef2b48ab..a1091b77 100644
--- a/src/tempo/TxEnvelopeTempo.ts
+++ b/src/tempo/TxEnvelopeTempo.ts
@@ -217,7 +217,10 @@ export function assert(envelope: PartialBy<TxEnvelopeTempo, 'type'>) {
   if (feeToken) Address.assert(feeToken, { strict: false })
 
   // Validate chain ID
-  if (chainId <= 0)
+  if (
+    typeof chainId !== 'undefined' &&
+    (!Number.isSafeInteger(chainId) || chainId <= 0)
+  )
     throw new TransactionEnvelope.InvalidChainIdError({ chainId })
 
   // Validate max fee per gas