diff --git a/packages/transactions/src/builders.ts b/packages/transactions/src/builders.ts
index e827bc1f6..c235f90cc 100644
--- a/packages/transactions/src/builders.ts
+++ b/packages/transactions/src/builders.ts
@@ -6,6 +6,7 @@ import {
   StacksTestnet,
   FetchFn,
   createFetchFn,
+  HIRO_TESTNET_DEFAULT,
 } from '@stacks/network';
 import { c32address } from 'c32check';
 import {
@@ -116,6 +117,37 @@ export async function getNonce(
   return BigInt(result.nonce);
 }
 
+/**
+ * Fetch the network chain ID from a given network. Typically used for looking up the chain ID for a non-default testnet network.
+ * @param useDefaultOnError - If the network fetch fails then return the default chain ID already specifed in the `network` arg.
+ */
+export async function getNetworkChainID(network: StacksNetwork, useDefaultOnError = true) {
+  const url = network.getInfoUrl();
+  try {
+    const response = await network.fetchFn(url);
+    if (!response.ok) {
+      const msg = await response.text().catch(() => '');
+      throw new Error(`Bad response status ${response.status} ${response.statusText}: "${msg}"`);
+    }
+    const responseJson: { network_id: number } = await response.json();
+    return responseJson.network_id;
+  } catch (error) {
+    if (!useDefaultOnError) {
+      throw error;
+    }
+    // log error and return default nework chain ID
+    console.warn(`Error fetching network chain ID from ${url}`, error);
+    return network.chainId;
+  }
+}
+
+function isNetworkCustomTestnet(network: StacksNetwork) {
+  return (
+    network.version !== TransactionVersion.Mainnet &&
+    new URL(network.coreApiUrl).host !== new URL(HIRO_TESTNET_DEFAULT).host
+  );
+}
+
 /**
  * @deprecated Use the new {@link estimateTransaction} function instead.
  *
@@ -751,6 +783,11 @@ export async function makeUnsignedSTXTokenTransfer(
     transaction.setNonce(txNonce);
   }
 
+  // Lookup chain ID for (non-primary) testnet networks
+  if (isNetworkCustomTestnet(network)) {
+    transaction.chainId = await getNetworkChainID(network);
+  }
+
   return transaction;
 }
 
@@ -1024,6 +1061,11 @@ export async function makeUnsignedContractDeploy(
     transaction.setNonce(txNonce);
   }
 
+  // Lookup chain ID for (non-primary) testnet networks
+  if (isNetworkCustomTestnet(network)) {
+    transaction.chainId = await getNetworkChainID(network);
+  }
+
   return transaction;
 }
 
@@ -1237,6 +1279,11 @@ export async function makeUnsignedContractCall(
     transaction.setNonce(txNonce);
   }
 
+  // Lookup chain ID for (non-primary) testnet networks
+  if (isNetworkCustomTestnet(network)) {
+    transaction.chainId = await getNetworkChainID(network);
+  }
+
   return transaction;
 }
 
diff --git a/packages/transactions/tests/builder.test.ts b/packages/transactions/tests/builder.test.ts
index cbf5cea2a..2da995c13 100644
--- a/packages/transactions/tests/builder.test.ts
+++ b/packages/transactions/tests/builder.test.ts
@@ -1,4 +1,5 @@
 import {
+  ChainID,
   PRIVATE_KEY_COMPRESSED_LENGTH,
   bytesToHex,
   bytesToUtf8,
@@ -332,6 +333,47 @@ test('Make STX token transfer with testnet string name', async () => {
   expect(serialized).toBe(tx);
 });
 
+test('Make STX token transfer with custom testnet (lookup network chainID)', async () => {
+  const nonce = 123;
+  const recipient = standardPrincipalCV('SP3FGQ8Z7JY9BWYZ5WM53E0M9NK7WHJF0691NZ159');
+  const amount = 12345;
+  const fee = 0;
+  const senderKey = 'cb3df38053d132895220b9ce471f6b676db5b9bf0b4adefb55f2118ece2478df01';
+  const senderAddress = 'STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6';
+  const memo = 'test memo';
+  const network = new StacksTestnet({ url: 'https://my-custom-testnet.example' });
+  const apiUrl = network.getAccountApiUrl(senderAddress);
+
+  fetchMock.mockRejectOnce();
+  fetchMock.mockOnce(`{"balance":"0", "nonce":${nonce}}`);
+
+  const fetchNonce = await getNonce(senderAddress, network);
+
+  fetchMock.mockRejectOnce();
+  fetchMock.mockOnce(`{"balance":"0", "nonce":${nonce}}`);
+
+  // http://localhost:3999/v2/info
+  fetchMock.once(JSON.stringify({ network_id: 0x1234 }));
+
+  const transaction = await makeSTXTokenTransfer({
+    recipient,
+    amount,
+    senderKey,
+    fee,
+    memo,
+    network,
+    anchorMode: AnchorMode.Any,
+  });
+
+  expect(fetchMock.mock.calls.length).toEqual(5);
+  expect(fetchMock.mock.calls[1][0]).toEqual(apiUrl);
+  expect(fetchMock.mock.calls[3][0]).toEqual(apiUrl);
+  expect(fetchMock.mock.calls[4][0]).toEqual(network.getInfoUrl());
+  expect(fetchNonce.toString()).toEqual(nonce.toString());
+  expect(transaction.auth.spendingCondition?.nonce?.toString()).toEqual(nonce.toString());
+  expect(transaction.chainId).toEqual(0x1234);
+});
+
 test('Throws making STX token transder with invalid network name', async () => {
   const txOptions = {
     recipient: standardPrincipalCV('SP3FGQ8Z7JY9BWYZ5WM53E0M9NK7WHJF0691NZ159'),
@@ -1242,6 +1284,9 @@ test('Estimate transaction fee fallback', async () => {
   // http://localhost:3999/v2/fees/transfer
   fetchMock.once('1');
 
+  // http://localhost:3999/v2/info
+  fetchMock.once(JSON.stringify({ network_id: ChainID.Testnet }));
+
   const tx = await makeContractCall({
     senderKey: privateKey,
     contractAddress: 'ST000000000000000000002AMW42H',
@@ -1291,7 +1336,7 @@ test('Estimate transaction fee fallback', async () => {
   const doubleRate = await estimateTransactionFeeWithFallback(tx, testnet);
   expect(doubleRate).toBe(402n);
 
-  expect(fetchMock.mock.calls.length).toEqual(8);
+  expect(fetchMock.mock.calls.length).toEqual(9);
 });
 
 test('Single-sig transaction byte length must include signature', async () => {