The 6-foot-10 Tillie said Wednesday that he loved "playing here and being a part of the amazing culture at GU."
Tillie's announcement comes three days after forward Rui Hachimura said he would return.
Tillie, who earned All-WCC honors and was the WCC tournament's most outstanding player, was second on the team in scoring (12.9), rebounding (5.8) and blocks (37).
He made 78 percent of his shots in three WCC tournament wins, including 13 of 14 3-pointers.
Gonzaga advanced to the Sweet 16 for the fourth straight season.