According to Peter Gammons of MLB.com and the MLB Network, Beltre has agreed to terms on a one-year deal for $9 million with a player option for $5 million, and that the deal is contingent on a physical.
Ken Rosenthal of FOXSports.com, citing a source, reported that Beltre's option for 2011 will increase to $10 million if he makes 640 plate appearances in '10.
The Red Sox have been in on Beltre since free agency began in November.
Though he is coming off a down year offensively with the Seattle Mariners, Beltre, 30, is considered an elite defender at third base, having won the Rawlings American League Gold Glove Award for his position in 2007 and '08.
Earlier this offseason, Red Sox general manager Theo Epstein added Marco Scutaro at shortstop and Mike Cameron in the outfield, two moves that will bolster the club's defense, which was regarded as one of the worst in the Majors in 2009. Boston's big free-agent signing was right-hander John Lackey, who should also play a crucial role in improving the team's run prevention.
And what about Mike Lowell, who has been Boston's primary third baseman since 2006? Last month, the Red Sox had a deal in place to send Lowell to the Texas Rangers, but the trade fell through for medical reasons.
Lowell underwent surgery last week to repair the radial collateral ligament in his right thumb. His recovery is expected to be six to eight weeks, putting the veteran in position to be ready near the start of Spring Training. It is unclear if the Red Sox will be able to trade Lowell until he proves his health in Grapefruit League games.
In the deal with the Rangers that fell through, the Red Sox would have sent a large sum -- believed to be $8 million to 9 million -- of the $12 million Lowell is due to earn in 2010.
With Beltre about to come on board, Kevin Youkilis can stay at first base, the position at which he won a Gold Glove Award in 2007. Though Youkilis is adept at both corner-infield spots, he is a little stronger at first.
Hindered by left shoulder woes in 2009, Beltre hit .265 with eight homers and 44 RBIs in 111 games and 477 at-bats.
The Red Sox hope that Beltre can get back to the consistent level Beltre enjoyed from 2006-08, when he averaged 25 homers and 88 RBIs. Playing half of his home games at Fenway Park instead of Safeco Field should help the right-handed-hitting infielder in that regard. Beltre is a pull hitter, which makes him well suited for Fenway, with the inviting Green Monster just 310 feet away.
Beltre had a breakout season with the Dodgers in 2004, hitting .334 with 48 homers and 121 RBIs, but he never approached anything like that before or after.
With the Red Sox, Beltre will be viewed as a complementary offensive player who can hit toward the bottom third of the order. Jacoby Ellsbury, Dustin Pedroia, Victor Martinez, Youkilis, David Ortiz and J.D. Drew will likely occupy the first six slots in some order.
In his career, split between the Dodgers and Mariners, Beltre is a .270 hitter with 250 homers and 906 RBIs.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.