Griffey had a two-run double and a two-run homer off Sergio Mitre (5-7), ending a slump of 52 at-bats since his last homer on Aug. 10. Griffey is sixth on the career homer list, trailing Barry Bonds, Hank Aaron, Babe Ruth, Willie Mays and Sammy Sosa.
Adam Dunn added a three-run homer in the seventh off reliever Byung-Hyun Kim, who signed as a free agent earlier in the day and was hit hard in his return to the floundering Marlins.
Florida has lost 10 of its last 11, tumbling a season-high 16 games under .500 at 57-73.
Right-hander Matt Belisle (7-8) struck out a career-high nine batters in seven-plus innings. He handed an 11-2 lead over to a bullpen that had another horrid eighth inning, allowing the Marlins to score six runs in all.
Before the game, baseball's first professional franchise gave Davey Concepcion its ultimate honor, retiring his No. 13. The Venezuelan shortstop anchored the Big Red Machine's infield, which included Pete Rose and Hall-of-Famers Joe Morgan, Tony Perez and Johnny Bench.
Ken Griffey Sr. was among the former teammates who were on the field before the game. Concepcion made a ceremonial pitch to Bench, then did a round of media interviews while Griffey's son led the Reds to their win.
Junior's 28th homer in the fifth inning snapped a streak of 52 at-bats without one, matching his longest of the season. He set the bat aside to give to Concepcion as a remembrance.
Since Pete Mackanin ran his first game as interim manager on July 3, the Reds have the best record in the NL at 28-19. The latest surge has been led by the offense, which has piled up 37 runs during the five-game winning streak.
Griffey had three hits off Mitre, who hasn't won since July 29. The right-hander is 0-2 with three no-decisions since then, giving up 24 runs in 28 2-3 innings.
Jeff Keppinger also had three hits for the Reds, extending his hitting streak to a career-high 14 games. Josh Hamilton had three singles and scored twice, giving him a career-high hitting streak of 10 games.
Belisle gave up eight hits, including solo homers by Jeremy Hermida and Dan Uggla. Miguel Olivo hit a three-run homer off Kirk Saarloos during the Marlins' eighth-inning rally.
Hermida had three hits and extended his hitting streak to a career-high nine games. He has homered in each of the last two. ... Kim will make a couple of relief appearances before getting a start. The right-hander gave up three hits and a walk in the seventh, including Dunn's 36th homer. ... RH reliever Ross Wolf was optioned to Triple-A Albuquerque. ... Concepcion was the eighth to have his number retired by the Reds, joining manager Fred Hutchinson (1), Bench (5), Morgan (8), manager Sparky Anderson (10), Ted Kluszewski (18), Frank Robinson (20) and Perez (24). Jackie Robinson's No. 42 has been retired by Major League Baseball.