He gets $8 million next season on the final year of a two-year deal that includes an $8 million team option for 2016. The Phillies included cash to pay part of Byrd's salary as part of the deal.
"Marlon will give us power and run production, and he can play left field," Reds general manager Walt Jocketty said. "We expect him to provide a big bat and balance in the middle of our lineup."
Philadelphia received minor league pitcher Ben Lively in return.
Byrd played center field and right field during his major league career, but came up through the minors as a left fielder.
"Hopefully it's going to be like riding a bike,'' Byrd said during a conference call. "I'm going to have to put a little extra work on it this offseason.''
He hit .264 with 25 home runs and 85 RBIs in 154 games with the Phillies last season.He also fanned a career-high 185 times.
"I know people are going to say he struck out a lot -- he struck out last year more than he has,'' Jocketty said. "He had 28 doubles, 25 homers and 85 RBIs, and that's something we definitely need on our club. Hopefully we can get him to cut down on the strikeouts.''
Phillies general manager Ruben Amaro Jr. already has traded away shortstop Jimmy Rollins (Los Angeles Dodgers) and reliever Antonio Bastardo (Pirates) this offseason. He is expected to continue exploring trades for pitcher Cole Hamels, first baseman Ryan Howard and other veteran players in the coming weeks as part of the team's rebuilding process.
The 22-year-old Lively made 26 starts last season at Class A Bakersfield and Class AA Pensacola. He combined to go 13-7 with a 3.04 ERA, and was named the organization's minor league player of the year.
"Yes, it's a tough decision to give up a young pitcher, but it really is a strength of our organization, especially right-handed pitching,'' Jocketty said. "They are paying a significant amount of his salary this year, which is the only way we would have been able to do this deal.''
The Associated Press contributed to this report.