Ambrose held off a late challenge by Joey Logano, finishing three seconds ahead and notching his third straight victory in the event.
He joins Terry Labonte as the only drivers to win three consecutive Nationwide races at Watkins Glen.
Keselowski's fourth-place showing allowed him to widen his lead over Carl Edwards to 327 points. Edwards had engine problems Saturday and wound up finishing 33rd.
Megyn Kelly: Santa Claus and Jesus are both white men
Wisconsin business offering 'therapeutic cuddling' forced to close