Florida is 5 1/2 games back of the New York Mets for the NL wild card with only six games left.
Joey Votto was 2-for-4 with a solo home run, two runs scored and two RBI and Andy Phillips added a pinch-hit two-run single for the Reds, who have won three of their last four games.
Aaron Harang (6-16) won his second straight start as he gave up four runs on eight hits in seven innings.
Jeremy Hermida and Josh Willingham each hit a solo home run for the Marlins, who have dropped three straight after a nine-game winning streak.
Florida starter Ricky Nolasco gave up three runs on six hits with a walk and five strikeouts in six-plus innings.
Andrew Miller (6-10) was charged with the loss.