The Niners (7-8) were forced to make a move because kicker Ricky Ricky Schmitt has a left groin injury and regular starter Joe Nedney has been sidelined with a hamstring injury. Schmitt was signed before last week's win over the Detroit Lions.
Coach Mike Singletary said Friday that San Francisco needed to find a kicker from outside the organization.
Andrus was released by Tampa Bay in early November. He appeared in one game for Indianapolis this season and three with the Buccaneers. He missed his lone field-goal attempt.
Andrus tried out with the Colts for the fourth straight year. He also was with the Colts in 2006 and 2007, but only lasted until training camp each time. He signed with the New York Giants in January 2008 but was waived last February after their Super Bowl win over New England and rejoined Indianapolis two weeks later.