Dellavedova's trey with 9 seconds remaining in regulation knotted the score at 62-62 and Mitchell Young scored four of his 12 points in the extra frame for the Gaels (27-5).
Brad Wardlow scored 23 points and hauled down 16 rebounds while Dellavedova finished with seven points and 12 assists as Saint Mary's advanced to Monday's WCC tourney championship tilt against No. 1 Gonzaga.
Gonzaga bested Loyola Marymount earlier Saturday in the other WCC semifinal.
Johnny Dee led the Toreros (16-18) with 18 points.