Milwaukee had a six-game winning streak snapped.
The score was tied at 73-73 when Stiemsma put the Celtics in front for good with a reverse layup. Mickael Pietrus scored four points during the run and Boston went on to capture its second straight and move within a half game of Philadelphia for the Atlantic Division lead.
Paul Pierce scored 25 points with nine rebounds to lead Boston. Rajon Rondo had 14 assists along with 10 points. Pietrus added 13 points off the bench.
Boston improved to 4-3 on a season-long, eight-game road trip that winds up Friday with a showdown against the 76ers for the division lead.
Brandon Jennings had 19 points for the Bucks and Carlos Delfino contributed 16. Monta Ellis, playing in Milwaukee for the first time since being traded to the Bucks from Golden State, had 13 points and seven assists.