A left knee injury hampered Iwamura this season, during which he played only 69 games and batted .290 with one home run, 16 doubles and 22 RBI.
Pirates general manager Neal Huntington issued a statement saying: "Akinori Iwamura brings to the Pittsburgh Pirates solid attributes offensively, defensively, on the base paths and in the club house. He is a good athlete with above-average speed and is a tough out with a solid career on-base percentage."
The 30-year-old Japan native has registered 14 homers, 67 doubles and 104 RBI over a 344-game career, all with Tampa Bay.
Chavez spent his first full major league season with the Pirates this year, going 1-4 with a 4.01 ERA in 73 games.