As Celtics fanatics — lowercase “f” — file into the TD Garden in Boston Thursday night for Game 5 of the Eastern Conference finals, some of them will have the option of betting with Fanatics — capital “F” — on their phone.

Fanatics Sportsbook, which was previously available in a limited capacity in Tennessee and Ohio, has entered its third state. The mobile app launched in Massachusetts Thursday for beta testing for select existing Fanatics customers.

As in Ohio and Tennessee, Fanatics Sportsbook will be rolled out gradually in Massachusetts. The first step is beta testing that is available by invitation only. Eligible customers on the Fanatics apparel mailing list will receive a code providing access to the mobile book.