Washington announced the deal Tuesday and said Stairs will be invited to the team's major league spring training camp. He turns 43 in February.
Stairs holds the career record with 23 pinch-hit homers, 15 more than any other active player. He has played for 12 big league teams and homered for 11, both tied for the career record.
In 18 major league seasons, he has a .264 batting average, 265 homers and 897 RBIs.
With the San Diego Padres last season, Stairs hit .232 with six homers and 16 RBIs in 99 at-bats. His four pinch-hit homers tied for the lead in the majors.