The Padres ended the Marlins' three-game winning streak, which had equaled their longest of the season.
Venable singled in a run in the second to tie it up and Forsythe hit his third home run of the season in the third to put the Padres in front for good. Venable added an RBI single during a three-run sixth and he had another one in the eighth.
Venable and Forsythe both had three hits as part of San Diego's total of 15.
Edinson Volquez (6-6) turned in a six-inning performance to get the victory. He allowed two runs on seven hits with eight strikeouts. The Padres had lost two straight.
Ricky Nolasco (4-8) was touched for five runs on 11 hits in five frames to suffer the loss.