Arenas, a second-round pick out of Alabama in 2010, has played in 63 games with 12 starts over his four-year career with Kansas City and Arizona. He also has served as a punt and kick returner.
Last season with the Cardinals, he had 12 tackles, one sack and returned 24 kickoffs for a career-best 21.4-yard average.
The Falcons are looking to bolster their secondary after releasing two starters, cornerback Asante Samuel and safety Thomas DeCoud.
Terms of deal with Arenas were not immediately available.