Venable's bases-clearing triple in third inning gave San Diego an 8-3 lead and the Padres outfielder then came home on a wild pitch. He finished with four RBI, while Carlos Quentin homered and drove in three for San Diego, which cruised to a victory in the opener of a four-game series.
Yonder Alonso went 3-for-4 with a run-scoring double for the Padres, who had 11 hits and drew 11 walks.
Edinson Volquez (7-7) stretched his unbeaten streak to seven games despite surrendering five runs on six hits over five innings.
Mike Leake (4-7) absorbed Cincinnati's first loss since July 18, lasting just 1 2/3 innings. The right-hander was roughed up for five runs on five hits.
Drew Stubbs and Jay Bruce knocked solo homers for the National League Central-leading Reds.