"I'm in preparation for a very incredible role," Usher said. "If you see me in the corners boxing, I'm doing that while working on this album."
Usher will star opposite Robert De Niro and Gael Garcia Bernal in the picture, which tells the story of another ring great, Roberto Duran.
De Niro is cast as boxing trainer Ray Arcell - who coached Duran to 18 world champions.
Leonard won championships in five different weight classes. He fought Duran on three separate occasions - winning twice.