Arizona had lost two straight and the Diamondbacks trailed by a run entering the seventh against the Padres.
The Diamondbacks then batted around against four San Diego pitchers to hand the Padres their third straight loss.
Martin Prado doubled home Montero with the first run of the seventh, Will Nieves had an RBI double and A.J. Pollock brought in another run with a single.
Paul Goldschmidt finished off the scoring with a two-run double.
San Diego starter Eric Stults (4-4) was tagged with the loss after giving up three of the seventh-inning runs.