diff --git a/src/transactions/index.ts b/src/transactions/index.ts
index 04d4874..979b73c 100644
--- a/src/transactions/index.ts
+++ b/src/transactions/index.ts
@@ -1,2 +1,25 @@
-// eslint-disable-next-line @typescript-eslint/no-explicit-any
-export function buildMultisigTransaction(..._args: unknown[]): unknown { return undefined; }
+import { Keypair, Transaction } from '@stellar/stellar-sdk';
+import { ValidationError } from '../utils/errors';
+import { isValidSecretKey } from '../utils/validation';
+
+export function buildMultisigTransaction(
+  transaction: Transaction,
+  secretKeys: string[],
+): Transaction {
+  if (!Array.isArray(secretKeys) || secretKeys.length === 0) {
+    throw new ValidationError('secretKeys', 'At least one secret key is required');
+  }
+
+  for (const secretKey of secretKeys) {
+    if (!isValidSecretKey(secretKey)) {
+      throw new ValidationError('secretKeys', 'Invalid secret key provided');
+    }
+  }
+
+  for (const secretKey of secretKeys) {
+    const signer = Keypair.fromSecret(secretKey);
+    transaction.sign(signer);
+  }
+
+  return transaction;
+}
diff --git a/tests/unit/transactions/index.test.ts b/tests/unit/transactions/index.test.ts
index e3de91c..245494e 100644
--- a/tests/unit/transactions/index.test.ts
+++ b/tests/unit/transactions/index.test.ts
@@ -1,8 +1,67 @@
 import { buildMultisigTransaction } from '../../../src/transactions';
+import { ValidationError } from '../../../src/utils/errors';
+import {
+  Account,
+  BASE_FEE,
+  Keypair,
+  Networks,
+  Operation,
+  Transaction,
+  TransactionBuilder,
+} from '@stellar/stellar-sdk';
 
-describe('transactions module placeholders', () => {
-  it('exports callable placeholder function', () => {
-    expect(buildMultisigTransaction()).toBeUndefined();
+function buildUnsignedTransaction(): Transaction {
+  const source = Keypair.random();
+  const account = new Account(source.publicKey(), '1');
+
+  return new TransactionBuilder(account, {
+    fee: BASE_FEE,
+    networkPassphrase: Networks.TESTNET,
+  })
+    .addOperation(Operation.bumpSequence({ bumpTo: '2' }))
+    .setTimeout(30)
+    .build();
+}
+
+describe('buildMultisigTransaction', () => {
+  it('signs transaction with a single secret key', () => {
+    const signer = Keypair.random();
+    const transaction = buildUnsignedTransaction();
+
+    const signed = buildMultisigTransaction(transaction, [signer.secret()]);
+
+    expect(signed).toBe(transaction);
+    expect(signed.signatures).toHaveLength(1);
+  });
+
+  it('signs transaction with multiple secret keys', () => {
+    const signerA = Keypair.random();
+    const signerB = Keypair.random();
+    const transaction = buildUnsignedTransaction();
+
+    const signed = buildMultisigTransaction(transaction, [signerA.secret(), signerB.secret()]);
+
+    expect(signed).toBe(transaction);
+    expect(signed.signatures).toHaveLength(2);
+  });
+
+  it('throws ValidationError when any secret key is invalid', () => {
+    const signer = Keypair.random();
+    const transaction = buildUnsignedTransaction();
+
+    expect(() => {
+      buildMultisigTransaction(transaction, [signer.secret(), 'SINVALID']);
+    }).toThrow(ValidationError);
+    expect(transaction.signatures).toHaveLength(0);
+  });
+
+  it('throws ValidationError when secret keys array is empty', () => {
+    const transaction = buildUnsignedTransaction();
+
+    expect(() => {
+      buildMultisigTransaction(transaction, []);
+    }).toThrow(ValidationError);
+    expect(transaction.signatures).toHaveLength(0);
   });
 });