The agreement is reported to be worth $24 million.
LaRoche led the Nationals with his 33 home runs, which was a personal best, and 100 RBI. The performance earned him a Silver Slugger Award. He also won his first Gold Glove after posting a .995 field percentage, committing only seven errors in 154 games.
LaRoche will be in his third season with the Nationals. He signed free agent contract with Washington in January 2011 and earned a total of $15 million the last two seasons.
In nine years in the major league, LaRoche has hit 268 with 197 home runs and 684 runs batted in. He previously played for Atlanta, Pittsburgh, Boston and Arizona.