Buffalo led 49-48 with 7:02 left before the Cougars used a 13-1 run to pull away. The Bulls went nearly six minutes without a field goal during Washington State's run.
Lacy tied his career high with five 3-pointers for Washington State (8-4), which has won three straight and six of seven. Motum had eight of his 15 points in the second half for the Cougars and Dexter Kernich-Drew added 10.
"If I'm going my teammates find me and if I'm not they'll find the person that's hot," said Lacy, who made four 3-pointers in the opening half.
Motum had two put-backs to ignite the run, including one after a missed free throw. Motum shot only 6 of 15 from the field and 1 of 4 on free throws. Motum had scored at least 23 points in each of the previous five games.
"He was one of the guys that was playing ugly but he was finding other ways to score and be productive," Washington State coach Ken Bone said about Motum.
Kernich-Drew hit a 3-pointer from the right wing to make it 56-50 with 4:53 remaining in the game.
"Dexter has not been making 3's," Bone said. "He had missed 10 in a row in the last three games, so it was important for us to work on him this last week to try to get him the confidence and freedom to shoot so it was like hope he makes it."
Tony Watson scored 18 points to lead Buffalo (4-8), which had won two in a row, and Auraum Nuiriankh added 13.
The Bulls shot 6 of 29 (20.7 percent) from the field in the second half. They shot 14 of 25 (56 percent) in the first half to lead 37-35 at halftime.
"We got out of sync," Buffalo coach Reggie Witherspoon said. "That happens if you get some bunnies and you miss them. Shots aren't going to get easier than that. A couple of them were four-point swings where we miss a layup and they come down and score."
Buffalo, which missed 9 of 10 shots to start the second half, finally began to heat up a little bit as Auraum Nuiriankh hit a 3-pointer to give the Bulls a 45-43 lead with 10:53 remaining.
Buffalo played without starting point guard Jarod Oldham, who injured his wrist at a Dec. 15 practice and is expected to miss the rest of the regular season. He was averaging 10.1 points and 4.4 assists per game. Freshman Jarryn Skeete replaced Oldham.
"It's hard," Witherspoon said about missing Oldham. "This was the first game for the freshman point guard on the road. I thought he didn't turn it over a lot but there was a lot of things coming at him in a quick way. We've just got to bring him along as fast as we can."
Skeete had nine points, no assists and two turnovers.
Watson scored 11 to help give Buffalo a 37-35 halftime lead. He had three 3-pointers during a 25-12 run to finish the half after the Cougars led 23-12 with 11:36 remaining.
Lacy scored 14 to lead Washington State in the opening half. Motum had seven points in the first half on 3-for-9 shooting from the field.
Both teams were 7 of 11 from beyond the arc in the first half.
This was the third game between the two schools but the first since 1950. The Cougars have won all three.