The team announced the multiyear deal on Tuesday without providing specific terms. Ellenson was taken with the No. 18 pick in last month's NBA draft.
Ellenson averaged 17 points and 9.7 rebounds per game last season as a freshman at Marquette. He also led the team with 49 blocked shots.
The 6-foot-11 Ellenson set school records for scoring and rebounding for a freshman.