diff --git a/modules/sdk-core/src/bitgo/wallet/BuildParams.ts b/modules/sdk-core/src/bitgo/wallet/BuildParams.ts
index cb62f470b9..54ba202ab7 100644
--- a/modules/sdk-core/src/bitgo/wallet/BuildParams.ts
+++ b/modules/sdk-core/src/bitgo/wallet/BuildParams.ts
@@ -158,6 +158,8 @@ export const BuildParams = t.exact(
       validToBlock: t.unknown,
       messageKey: t.unknown,
       stakingParams: t.unknown,
+      // PoX-5 (UTXO) unstaking, e.g. { pox5: { branch: 'early-exit', principalPreimage } }
+      unstakingParams: t.unknown,
       stakingOptions: t.unknown,
       unstakingOptions: t.unknown,
       eip1559: t.unknown,
diff --git a/modules/sdk-core/src/bitgo/wallet/iWallet.ts b/modules/sdk-core/src/bitgo/wallet/iWallet.ts
index 107962047e..fdf54b1770 100644
--- a/modules/sdk-core/src/bitgo/wallet/iWallet.ts
+++ b/modules/sdk-core/src/bitgo/wallet/iWallet.ts
@@ -241,6 +241,10 @@ export interface PrebuildTransactionOptions {
   custodianTransactionId?: string;
   apiVersion?: ApiVersion;
   stakingParams?: unknown;
+  /**
+   * PoX-5 unstaking parameters, e.g. `{ pox5: { branch: 'early-exit', principalPreimage } }`.
+   */
+  unstakingParams?: unknown;
   /**
    * If set to false, sweep all funds including the required minimums for address(es). E.g. Polkadot (DOT) requires 1 DOT minimum.
    */
diff --git a/modules/sdk-core/src/bitgo/wallet/wallet.ts b/modules/sdk-core/src/bitgo/wallet/wallet.ts
index 3a37110801..2eebc77efe 100644
--- a/modules/sdk-core/src/bitgo/wallet/wallet.ts
+++ b/modules/sdk-core/src/bitgo/wallet/wallet.ts
@@ -160,6 +160,11 @@ type ManageUnspents = 'consolidate' | 'fanout';
 
 const whitelistedSendParams = TxSendBody.type.types.flatMap((t) => Object.keys(t.props));
 
+// TxSendBody in @bitgo/public-types does not declare unstakingParams yet, so /tx/initiate would
+// drop it. Extend locally until the codec has it.
+const TxInitiateBody = t.intersection([TxSendBody, t.partial({ unstakingParams: t.unknown })]);
+const whitelistedInitiateParams = [...whitelistedSendParams, 'unstakingParams'];
+
 export enum ManageUnspentsOptions {
   BUILD_ONLY,
   BUILD_SIGN_SEND,
@@ -5782,17 +5787,17 @@ export class Wallet implements IWallet {
     ).result();
   }
 
-  private initiateTransaction(params: TxSendBody, reqId?: IRequestTracer) {
+  private initiateTransaction(params: t.TypeOf<typeof TxInitiateBody>, reqId?: IRequestTracer) {
     // extract the whitelisted params from the top level, in case
     // other invalid params are present that would fail encoding
     // and fall back to the body params
-    const whitelistedParams = this.baseCoin.preprocessBuildParams(_.pick(params, whitelistedSendParams));
+    const whitelistedParams = this.baseCoin.preprocessBuildParams(_.pick(params, whitelistedInitiateParams));
     const reqTracer = reqId || new RequestTracer();
     this.bitgo.setRequestTracer(reqTracer);
     return postWithCodec(
       this.bitgo,
       this.baseCoin.url('/wallet/' + this.id() + '/tx/initiate'),
-      TxSendBody,
+      TxInitiateBody,
       whitelistedParams
     ).result();
   }
diff --git a/modules/sdk-core/test/unit/bitgo/wallet/custodialInitiate.ts b/modules/sdk-core/test/unit/bitgo/wallet/custodialInitiate.ts
new file mode 100644
index 0000000000..783827f519
--- /dev/null
+++ b/modules/sdk-core/test/unit/bitgo/wallet/custodialInitiate.ts
@@ -0,0 +1,56 @@
+import * as assert from 'assert';
+import sinon from 'sinon';
+import { Wallet } from '../../../../src';
+
+describe('Wallet - custodial tx/initiate', function () {
+  const unstakingParams = {
+    pox5: { branch: 'early-exit', principalPreimage: '00'.repeat(32) },
+  };
+  const recipients = [{ address: 'tb1qexample', amount: 'max' }];
+
+  let wallet: Wallet;
+  let mockBitGo: any;
+  let sendStub: sinon.SinonStub;
+
+  beforeEach(function () {
+    sendStub = sinon.stub().returns({ result: sinon.stub().resolves({ status: 'accepted' }) });
+    mockBitGo = {
+      post: sinon.stub().returns({ send: sendStub }),
+      setRequestTracer: sinon.stub(),
+    };
+    const mockBaseCoin = {
+      url: sinon.stub().callsFake((path: string) => `/tbtcstxsignet${path}`),
+      checkRecipient: sinon.stub(),
+      preprocessBuildParams: sinon.stub().callsFake((params: unknown) => params),
+      getExtraPrebuildParams: sinon.stub().resolves({}),
+      supportsTss: sinon.stub().returns(false),
+    };
+    wallet = new Wallet(mockBitGo, mockBaseCoin as any, {
+      id: 'test-wallet-id',
+      keys: ['user-key'],
+      type: 'custodial',
+      multisigType: 'onchain',
+    });
+  });
+
+  afterEach(function () {
+    sinon.restore();
+  });
+
+  it('forwards unstakingParams from sendMany to the tx/initiate request body', async function () {
+    await wallet.sendMany({ recipients, txFormat: 'psbt', noSplitChange: true, unstakingParams } as any);
+
+    assert.strictEqual(mockBitGo.post.callCount, 1);
+    assert.strictEqual(mockBitGo.post.firstCall.args[0], '/tbtcstxsignet/wallet/test-wallet-id/tx/initiate');
+    const body = sendStub.firstCall.args[0];
+    assert.deepStrictEqual(body.unstakingParams, unstakingParams);
+    assert.deepStrictEqual(body.recipients, recipients);
+    assert.strictEqual(body.txFormat, 'psbt');
+  });
+
+  it('does not add unstakingParams when the caller did not set it', async function () {
+    await wallet.sendMany({ recipients } as any);
+
+    assert.ok(!('unstakingParams' in sendStub.firstCall.args[0]));
+  });
+});