The deal, agreed to a week earlier subject to a physical, calls for a $1.25 million salary this year and includes a $2 million club option for 2011 with a $500,000 buyout. Kennedy took his physical Friday.
The 34-year-old hit .289 with 11 homers and 63 RBIs for the Oakland Athletics last season, his 11th in the majors.
He provides a defensive upgrade at second base for the Nationals.
"A couple of the guys that we do have on the staff are groundball pitchers, and we are going to be counted on to help them," Kennedy said in a telephone interview. "So, hopefully, I can help with that."
Kennedy's acquisition probably means Cristian Guzman will stay at shortstop. There had been some discussion of moving him to second base.
Kennedy has a career .277 batting average, with 68 homers and 486 RBIs while playing for the Cardinals, Angels and Athletics. He was voted MVP of the 2002 American League championship series with the Angels.
While the Nationals lost more than 100 games each of the past two seasons, Kennedy took note of general manager Mike Rizzo's roster moves.
"He's got some pretty nice players," Kennedy said. "I like the direction he seems be having the team going."
Washington also has added catcher Ivan Rodriguez, starting pitcher Jason Marquis, closer Matt Capps and relievers Brian Bruney, Tyler Walker and Eddie Guardado.
Pitchers and catchers report for Nationals spring training at Viera, Fla., on Feb. 19.
"I just think we're all looking forward to getting down to Florida and getting familiar with each other," Kennedy said, "and getting ready to make a run at it."
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