Duvall was targeted for a platoon role, starting against left-handers. The move will cut into Ender Inciarte's playing time.
The Braves sent right-handers Lucas Sims and Matt Wisler and outfielder Preston Tucker to Cincinnati. All three spent time in Atlanta this season but were with Triple-A Gwinnett at the time of the trade.
The trade was announced following the Braves' 5-3 win over Miami on Monday night.
Duvall, 29, has hit 15 home runs with 61 RBIs but is hitting only .205 this season.
Braves general manager Alex Anthopoulos said the Braves plan to start Duvall in left field against left-handers, with rookie Ronald Acuna Jr. moving to center and Inciarte, a Gold Glove Award-winning center fielder, moving to the bench.
''We've been a good team against left-handers but we can get better and this is a guy who's under control going forward,'' Anthopoulos said.
Anthopoulos said Duvall and Inciarte have been told of the outfield plan.
''I talked to Ender tonight and he was great,'' Anthopoulos said. ''His response was `I just want to get to the playoffs.' He's awesome.''
Duvall hit 64 homers over the previous two seasons, including 31 in 2017. He was an NL All-Star in 2016.
Duvall was also a finalist for the Gold Glove Award in each of the last two seasons.
The Braves are only a half-game behind Philadelphia in the race for the NL East. The Reds are last in the NL Central despite a recent surge.
''We're pleased with the performance we've seen in the middle of the summer,'' said Reds president of baseball operations Dick Williams. ''We've been trying to keep an eye on next year but this year is also important. ... I think when we looked at next year we saw Winker and Schebler really emerging as corner outfielders that are going to have a lot of success in the big leagues.''
Wisler, 25, had a 5.40 ERA in seven games, including three starts, with Atlanta this season. Sims, 24, had a 7.84 ERA in six relief appearances. Tucker, 28, hit .256 with four homers and 22 RBI in 62 games. Tucker was optioned to Gwinnett on Saturday.
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