Updated at 10:38 p.m.
The deal won't become official until LaRoche passes a physical, which is due to take place Thursday in Washington. Contract terms stipulate that LaRoche will make $7 million in 2011 and $8 million in 2012, with a $10 million mutual option for 2013 that the Nationals can buy out for $1 million.
The 31-year-old has been on the Nationals' radar for nearly a month, ever since the club found itself in need of a new first baseman after Adam Dunn signed a four-year, $56 million contract with the White Sox. General manager Mike Rizzo also pursued fellow free agents Carlos Pena and Derrek Lee, but each veteran signed elsewhere (Pena with the Cubs, Lee with the Orioles).
That left LaRoche as the lone remaining free agent first baseman of significance, and the Nationals as the lone remaining team still in the market for such a player.
LaRoche is a career .271 hitter over seven big-league seasons with the Braves, Pirates, Red Sox and Diamondbacks. He has hit exactly 25 home runs each of the last three years and set a career high with 100 RBI last season with Arizona (though his .261 batting average, .320 on-base percentage and .468 slugging percentage all fell below his career averages).
LaRoche's offensive production won't match the man he's replacing -- Dunn has hit at least 38 homers with at least a .356 on-base percentage each of the last seven years -- but he is a more gifted defensive player. His .995 career fielding percentage is fifth-best among all qualifying first basemen since 2004. (His Ultimate Zone Rating over the span, though, is -15.9, fourth-worst among qualifying first basemen, according to Fangraphs.)
The left-handed LaRoche figures to bat either fourth or fifth, joining Ryan Zimmerman and Jayson Werth as the heart of the Nationals' new-look lineup.