Nike had been making the club's shirt since 2013 and New Balance becomes the sixth supplier in the history of the club.
The six-season deal will run from 2017/18 to 2022/23. New Balance will provide clothing to each of the club’s teams.
Richard Wright, general manager of New Balance Football, said: "We are delighted that Athletic Club has joined New Balance Football, we are looking forward to working closely with a great club with such fantastic traditions. As well as celebrating the club's heritage, we will bring innovation and world class product to the club and its fan base. We're certainly looking forward to unveiling our first kit collection very soon."