With the score tied 1-1 entering the frame, Fielder's homer off Brett Anderson (1-2) put the Tigers ahead to stay.
Then, after Victor Martinez singled and Matt Tuiasosopo drew a walk, Peralta belted another round-tripper, breaking the contest open.
Torii Hunter also went deep for Detroit, which evened its three-game road series.
Justin Verlander (2-1) yielded just one run over six innings to record the victory. The Tigers ace allowed three hits, walked three and struck out six.
Anderson was torched for seven runs on eight hits over 5 2/3 frames for the Athletics, whose nine-game winning streak was snapped.
Chris Young drove in a pair of runs for Oakland.