Matusz (4-4) won his third consecutive start, taming the Red Sox on two hits while allowing one run and issuing a single walk.
With Baltimore clinging to a 2-1 lead in the eighth, Matt Wieters drew a one-out walk from Matt Albers and Betemit blasted long homer to right, sealing the victory.
Steve Tolleson also homered for Baltimore.
Boston starter Felix Doubront (4-2) lost despite striking out nine over six innings. He gave up a pair of runs on four hits and two walks.
Kevin Youkilis celebrated his return from the disabled list with solo homer in the fourth.