The deal includes a $2 million club option in 2011 for the former Oakland Athletics player, the Nationals reported on their website (http://washington.nationals.mlb.com).
"I really like Adam a lot as a player," manager Jim Riggleman said. "He isn't spectacular in any particular area of the game. He is just a solid pro."
The 34-year-old hit .289 with 11 home runs, 63 runs batted in and 20 stolen bases for the Athletics last season. He has also played with St. Louis and the Los Angeles Angels and was the American League Championship Series MVP in 2002.
(Reporting by Gene Cherry in Salvo, North Carolina)