The shirt is meant as a show of support for the family of Eric Garner, who died July 17 after a police officer placed him in a choke hold during an arrest. A recording of Garner's arrest showed him gasping “I can't breathe” during the encounter.
A New York grand jury decided not to indict NYPD officer Daniel Pantaleo in Garner's death, and thousands have protested since that decision was announced last Wednesday.
Social media lighted up with a picture of James wearing the shirt; there had been speculation this would be the second time he publicly took a stand on racial issues. The first was when he joined other Miami Heat players in wearing hoodies after the fatal shooting of Trayvon Martin.
Chicago Bulls point guard Derrick Rose wore an "I Can't Breathe" shirt during warmups before Saturday's home game against the Golden State Warriors.
James said Sunday that he wanted one of the shirts, and he got one from Nets guard Jarrett Jack, who provided shirts to players of both teams before Monday's game at Brooklyn. Cavaliers guard Kyrie Irving and the Nets' Kevin Garnett and Deron Williams were among players who wore the shirts during warmups.
NFL players, including the Detroit Lions' Reggie Bush, also have worn the shirt.