The 31-year-old, who impressed during pre-season training with the club, knows manager Sven Goran Eriksson both from his time at Manchester City and his one appearance for England in 2001.
"Michael is another very experienced player that will add further depth to our squad," Eriksson told the club's official website. "He is a good influence both on and off the pitch and has a positive effect in the dressing room.
"He has come back from a long spell out injured and has a good opportunity to recapture his previous form. If he can do that, he will be a bargain for us."
Ball has been without a club since 2009, after he was released by Manchester City.