BOSTON — The Boston Red Sox began the offseason hoping to improve their defense. They did it again by reaching a tentative agreement with third baseman Adrian Beltre.

Beltre and the Red Sox have agreed on a one-year contract, according to a person familiar with the negotiations. The person spoke to The Associated Press on condition of anonymity Tuesday because the agreement is subject to a physical and is not yet final.

Beltre, who spent the past five seasons with Seattle, joins shortstop Marco Scutaro and Gold Glove outfielder Mike Cameron as free agents who landed in Boston this offseason.

Barring other moves, the offense figures to be weaker with the departure of Jason Bay, who led the team last year with 36 homers and 119 RBIs, to the New York Mets as a free agent.

Beltre would be guaranteed $10 million, another person said, also speaking on condition of anonymity because the deal was not final. Beltre would get $9 million this year with a $5 million player option for 2011 with a $1 million buyout. The option could escalate to $10 million

With Beltre joining first baseman Kevin Youkilis and second baseman Dustin Pedroia, the Red Sox have three infielders who have won Gold Gloves. Cameron, likely to start in center or left field, has three by himself.

"A lot of talk has been centered on our offense last year and how to fix our offense," general manager Theo Epstein said on Dec. 16 when the Red Sox held news conferences to introduce Cameron and pitcher John Lackey as free-agent pickups, "but lost in that mix is that our run prevention needs some improvement as well. We were one of the worst defensive clubs in baseball last year."

Beltre won Gold Gloves in 2007 and 2008 and is an upgrade at third over Mike Lowell, whose solid defense declined after he underwent hip surgery before last season.

Boston had worked out a trade to send Lowell to the Texas Rangers for catcher Max Ramirez but that was derailed last month when an examination showed Lowell had an injured thumb. He had surgery on Dec. 30 and is expected to be ready for spring training.

The Red Sox figure to continue to try to trade Lowell. If they don't, he could share time at designated hitter with David Ortiz.

Beltre was limited to 111 games last season by injuries to his shoulder and testicle. That was the fewest since he was a rookie with the Los Angeles Dodgers in 1998. He played in at least 143 games every season from 2002 through 2008.

His best offensive season was 2004, his last with the Dodgers, when he hit .334 with 48 homers and 121 RBIs. The next season he started his five-year, $64 million contract with the Mariners, who play in spacious Safeco Field.

With Seattle, he hit .254 with 48 homers and 174 RBIs at home and .277 with 55 homers and 222 RBIs on the road. In 1,681 career games, Beltre has hit .270 with 250 homers and 906 RBIs.