Marshall scored 17 points and grabbed 10 rebounds for the fifth-seeded Bulldogs (20-13), who will meet top-seeded Valparaiso in the semifinals Saturday night. Roosevelt Jones made 8 of 11 shots and matched Marshall’s 17 points.
James Haarsma’s nine points led the fourth-seeded Panthers (20-13), whose five-game winning streak ended. Milwaukee shot 32 percent and was outrebounded 46-25.
Butler, which has won seven of eight, had two 11-0 runs.
The Bulldogs led 20-18 before scoring the next 11 points, seven by Marshall, for a 31-18 lead with 17:30 left.
After the Panthers closed within 33-26, Butler reeled off another 11 in a row for a 44-26 edge with 11:47 remaining.
It got no closer than 13 after that.
Butler clinched its seventh consecutive 20-win season.