Drummond will travel to Philadelphia for a one year deal with the league minimum. Niang will sign a $6.7 million two-year contract.
Drummond, who will turn 28 next Tuesday, was waived by the Cleveland Cavaliers and joined the Los Angeles Lakers in march. In 21 regular season games, he averaged 11.9 points with 10.2 rebounds. He also scored nine points and 11 boards in the Lakers' playoff loss to Suns.
A two-time All-Star with the Detroit Pistons, Drummond was named third-team All-NBA in 2015-16. Drummond has been the league's leader in rebounding four times.
Niang, 28, joins the Sixers after four seasons with the Utah Jazz. He played in 72 of the regular season's 72 games, scoring 6.9 points per game and shooting 42.5 percent from three-point range.