Brad Waldow added 17 points and Stephen Holt scored 13 with six assists and five rebounds for the Gaels (16-2, 5-0 WCC), who have won six straight games.
Elias Harris posted a double-double with 17 points and 11 rebounds, while Kevin Pangos donated 12 points for Gonzaga (13-3, 3-1), which saw its eight- game winning streak come to an end.
The two teams felt each other out over the first 10 minutes, exchanging baskets and playing to a 15-15 draw. Saint Mary's opened a 24-18 lead on Mitchell Young's layup, but the Bulldogs roared back to a stalemate at 29-29 on Guy Landry Edi's tip off his own miss with just under five minutes to go.
That tip represented Gonzaga's final points of the half. The Gaels scored the final eight with Dellavedova's free throw closing out the scoring at 37-29.
The entire second half must have felt like the final five minutes of the first for Gonzaga, which was got in the way of Dellavedova's scoring onslaught. The margin quickly grew to double digits at 48-37 thanks to Clint Steindl's three, and Dellavedova's trey a few minutes later made the score 59-45.
The advantage grew even higher when Dellavedova drained another triple, his one from the left wing for a 69-52 lead. He wasn't done, as another not even two minutes later kept the 17-point margin, 72-55.