Johnson played 10 games for the Rays in 2008, batting .200 with two homers and four RBIs. He spent last season with the Yokohama BayStars of the Japanese Central League, finishing with a .215 average, 24 homers and 57 RBIs.
Johnson, 30, was selected by Oakland in the seventh round of the 2001 draft. He has a .248 career average with 44 homers and 161 RBIs in parts of four seasons with the Athletics and Rays.
He is guaranteed $175,000. Johnson can earn $50,000 in performance bonuses: $10,000 each for 300, 350, 400, 450 and 500 plate appearances.
Tampa Bay announced the deal on Monday.