Nick Perkins had 16 points and the Bulls (24-8) made two big 3-pointers in the final 2:17 to put away the Chippewas.
Buffalo plays fifth-seeded Kent State, the defending tournament champion, in Friday's first semifinal.
Cecil Williams had 21 points and 13 rebounds to pace Central Michigan (19-14), which cut a 14-point second-half deficit to two, but couldn't come up with a big defensive stop in the final minutes.
The Chippewas were still within 77-74 when Wes Clark knocked down a 3-pointer for Buffalo. After Williams was called for a charging foul on the next possession, Massinburg, who had missed three free throws, drilled his 3 from the left wing.
Buffalo entered the MAC tourney as the No. 1 seed for the first time. The Bulls are seeking their third title in four years after winning in 2015 and 2016.
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