Penny joins a rotation that includes Tim Lincecum and Barry Zito for a team that is tied with the Colorado Rockies for the National League wild card.
Penny's year started out well in Boston, when he went 6-2 with a 4.94 ERA through his first 13 starts. He went winless in his final five starts, however, going 0-4 with a 9.11 ERA.
Penny is expected to make his Giants debut on Wednesday against the Phillies. For Penny, it's not his first experience in the NL West, as he spent parts of five seasons pitching for the Los Angeles Dodgers. Because he joined the team before the end of Aug. 31, he is eligible to be on the postseason roster.
