The left-hander can earn another $1.2 million in performance bonuses.
Howell, 29, came to the Rays in a 2006 trade that sent outfielder Joey Gathright and infielder Fernando Cortez to the Royals.
Howell went 1-0 with a 3.04 ERA in 55 games in 2012, enjoying a fully healthy year after missing all of the '10 season and the first 44 games of '11 due to a left shoulder injury.
Howell established a club scoreless streak of 27 1/3 innings by not allowing a run from June 14 until Aug. 30.
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.