With the Astros clinging to a 2-0 lead with one out in the fifth inning, Jose Altuve and Tyler Greene singled and Wallace followed by blasting a pitch into the right-center field bleachers.
Houston starter Edgar Gonzalez (1-0) allowed one run over 5 1/3 innings in only his second appearance since 2009. The right-hander surrendered five hits, issued one walk and struck out five.
Jeff Locke (0-1) started for the Pirates and was roughed up for five runs on seven hits over five full frames as Pittsburgh dropped a fourth straight.
Clint Barmes drove in the only run for the Bucs, who fell 2 1/2 games behind St. Louis for a National League wild card spot.