Yabu, 41, was released by the San Francisco Giants in July 2009 when he was at Triple-A Fresno.
Previously in Japan, he was 84-106 with a 3.57 ERA with the Hanshin Tigers in 1994-2004.
In the major leagues, Yabu made 40 appearances for the Oakland Athletics in 2005 and pitched in 60 games for San Francisco in 2008, all in relief.
He tried out with Rakuten on Thursday and Friday after a failed tryout with the Lotte Marines earlier this week.