Brad Waldow scored 19 points and Clint Steindl added 16 off the bench to lead No. 21 St. Mary's to an 80-66 win over BYU on Saturday night.
The Gaels (21-3, 10-0 West Coast) also got 13 points and nine rebounds from Rob Jones.
St. Mary's leading scorer Matthew Dellavedova had 10 points.
Steindl scored 14 points in the first half when he hit his first four 3-point shots. He finished 4 of 6 from behind the arc.
The Cougars (18-6, 6-3) were led by Noah Hartsock and Matt Carlino, who scored 15 points. Brandon Davies added 14 points and seven rebounds.
St. Mary's used a 10-0 run to build a 40-28 halftime lead, and stretched the margin to 23 points early in the second half.
BYU cut the lead to nine, but could never get closer.
St. Mary's and BYU spent most of the first half using their inside game.
The difference in the opening 20 minutes, however, was Steindl. With the game tied 13-13, Steindl came off the bench to hit his first 3 from the right corner to give the Gaels the lead.
The Cougars got back to 22-20 on a Hartsock score, but Waldow then broke loose for seven points in the paint and another Steindl 3 pushed St. Mary's to a 32-20 lead.
After taking a 12-point lead into the break, St. Mary's widened the margin to 55-32 at the 16:49 mark of the second half when Dellavedova and Jorden Page each hit a 3 and Waldow added two more baskets in the paint.
The Cougars went on a 15-5 run to cut the St. Mary's lead to 60-47, but the teams traded baskets for the next several minutes to keep the Gaels' margin at 13.
Carlino scored five straight to make it 70-61.
St. Mary's was able to hang on, however, when BYU turned the ball over on two straight possessions and the Gaels hit 8 of 10 free throws down the stretch to finish things off.
The Gaels shot 56 percent from the floor to only 41 percent for the Cougars.
St. Mary's hit 7 of 18 shots from 3-point range, with BYU hitting only 2 of 13 long-range shots.
The Gaels also made 25 of 33 from the line. The Cougars were 14 of 22 from the line. St. Mary's won despite turning the ball over 24 times.