The four-year pro was drafted in the third round by the Ravens in 2008 and spent his entire career there.
Last season, he played in all 16 games and had 19 tackles.
"Tom is a very dedicated and fiery player," said Colts head coach Chuck Pagano. "His competitiveness drives him to perform at a high level and his aggressive style of play parallels the attitude we're looking for from the defensive unit."
In 53 games with the Ravens -- 14 starts -- he has notched 49 tackles, two interceptions, one sack and five passes defensed. Also, he has returned 14 kickoffs for a 21.8 yards average and 16 punts for 6.3 yards.