Beltre's contract is subject to him passing a physical, a person close to the third baseman said, speaking on condition of anonymity.
The deal could be announced as early as Friday. Seattle did not comment on the agreement.
A day earlier, the Mariners added another slugger by finalizing a $50 million, four-year contract with free-agent first baseman Richie Sexson.
Seattle finished a distant last in the AL West, going 63-99 for its worst record since 1983. The Mariners were last in the league with only 136 homers and 698 runs.
Beltre also improved on defense last season, committing only 10 errors.
The Dodgers wanted to keep Beltre and offered him salary arbitration. But Los Angeles recently gave itself some protection, signing free agent Jeff Kent and saying that he could play third base, along with his usual spot at second.
"We're disappointed. It hurts," Dodgers general manager Paul DePodesta said. "Obviously, we would have loved to have him back.
"I think all of us knew this certainly was a possibility. We feel like we stepped up and made a very competitive offer, actually tried to make it a compelling one by offering a sixth guaranteed year. Unfortunately for us, Adrian decided to go elsewhere."
Beltre made $5 million last season and figured to cash in after a career year in which he led the Dodgers to the NL West division title and their first postseason spot since 1988.
DePodesta said the Dodgers' six-year offer to Beltre and agent Scott Boras included an option for a seventh season.
When asked if the Dodgers were given a final say in the matter, DePodesta said: "Not in so many words. They didn't come back and say, 'If you can do this, he's coming back.' "
"Scott did have a sense of urgency, we did try to step up to that. That really wasn't the forum. But then again, I'm not blaming Scott for that, either. Everything he did in this process was above board. There's no villain here," he said. "At the end of the day, Seattle stepped up and made a great offer. We had multiple meetings, face-to-face meetings."
Beltre became a free agent after the season, filing on the same day he had arthroscopic surgery to remove two large bone spurs from his left ankle.
At the time, the Dodgers said Beltre was expected to complete his recovery from ankle surgery before spring training. He was expected to be able to resume jogging and running in eight weeks.
Beltre made his major-league debut in 1998 at 19. He had posted career highs of 23 home runs, 85 RBI and a .290 batting average before breaking loose this season.
Beltre had 200 hits and 104 runs and improved his walks-to-strikeouts ratio. In 2003, he batted .240 and struck out 103 times.
The Mariners tried a lot of people at third base last season. Scott Spiezio led the team with 65 starts at the position and batted just .215.