Free-agent first baseman Adam LaRoche and the Washington Nationals agreed to a two-year contract, pending a physical, a person familiar with the negotiations told The Associated Press.

Speaking on condition of anonymity Tuesday night because the deal had not been completed, the person said the agreement contains a mutual option for 2013.

LaRoche would fill a big need for the Nationals, who lost first baseman and cleanup hitter Adam Dunn to the Chicago White Sox via free agency this offseason. Like Dunn, LaRoche bats left-handed, and he would join a middle of the lineup that includes new arrival Jayson Werth and third baseman Ryan Zimmerman, who both are right-handed hitters.

The 31-year-old LaRoche hit .261 with 25 homers and a career-high 100 RBIs for the Arizona Diamondbacks last season, when he struck out 172 times. He has a career .271 average with 161 homers and 569 RBIs during seven major league seasons with four clubs, including the Atlanta Braves, Pittsburgh Pirates and a brief stint with the Boston Red Sox.

While he doesn't provide the same power as Dunn — who hit at least 38 homers each of the past seven seasons and drove in at least 100 runs in six of those years — LaRoche represents an upgrade defensively. He has a .995 career fielding percentage.

LaRoche is the second significant addition of the offseason for the Nationals, who are coming off three consecutive last-place finishes in the NL East. Right fielder Werth signed a seven-year, $126 million contract to come over as a free agent from the Philadelphia Phillies, a deal announced at the outset of baseball's winter meetings last month.

The addition of LaRoche should complete Washington's starting infield, which is expected to include Ian Desmond at shortstop and Danny Espinosa at second base.

LaRoche is the older brother of Pirates third baseman Andy LaRoche and son of former major league pitcher Dave LaRoche.