Byrd, 37, is hitting .237 on the year with 19 home runs, 31 RBIs and a .743 OPS in his 96 games with the Reds. Fifteen of Byrd's 19 home runs have come off of right-handed pitchers this season.
The Giants will use Byrd to help offset injuries to outfielders Hunter Pence (strained oblique) and Angel Pagan (patella tendonitis), who are on the disabled list. Nori Aoki and Gregor Blanco are the other outfielders in the Giants lineup.
San Francisco enters Thursday night with a 65-55 record and is just two games behind the Los Angeles Dodgers for first place in the National League West. The Giants are also three games out in the wild card chase.
Byrd, who will be a free agent at the end of the season, is expected to join the team before the Giants' Friday night game against the Pittsburgh Pirates at PNC Park.
- Christopher Chavez