The person spoke on condition of anonymity Thursday because LaRoche had to complete the physical needed before the contract can be finalized.
The 30-year-old left-handed batter hit .277 for Atlanta, Boston and Pittsburgh last season with 25 home runs and 83 RBIs. He has a .274 career average with 136 homers in six seasons.
Last week, LaRoche reportedly turned down a two-year, $17 million offer from the San Francisco Giants.
Adding LaRoche would allow Arizona to shift Conor Jackson to his more natural outfield position. Jackson missed virtually all of last season with an illness known as "valley fever."
The team would have Jackson in left, Chris Young in center and All-Star Justin Upton in right, with fourth outfielder Gerardo Parra able to play all three positions.
The Diamondbacks, who finished last in the NL West at 70-92 last year, have had a relatively busy offseason.
Earlier, they signed second baseman Kelly Johnson to a one-year contract.
They also were part of a three-team trade that brought starting right-handers Edwin Jackson from Detroit and Ian Kennedy from the New York Yankees. Arizona gave up right-hander Max Scherzer and left-handed prospect Daniel Schlereth.
Arizona signed right-handed reliever Bob Howry to a one-year, $2.25 million contract with a club option for 2011.
LaRoche played for three teams last season. He started the year with Pittsburgh, then was traded to Boston for two minor leaguers on July 22. He played just six games for the Red Sox before being traded to Atlanta for Casey Kotchman on July 31.
LaRoche started his major league career with the Braves and spent three seasons with Atlanta before he was traded to Pittsburgh in January 2007 in a deal that brought reliever Mike Gonzalez to the Braves. LaRoche's brother Andy plays for the Pirates.
LaRoche's best season was 2006, when he hit .285 with 32 home runs and 90 RBIs, all career highs. His numbers are far better in the second half of the season than the first.
Ayala reveals River wishAtlanta Braves, Troy Glaus finalize 1-year, $1.75M contract
