Beltre passed a physical, the team's final step before securing the slugging third baseman.
"Adrian is one of the best young position players in baseball," general manager Bill Bavasi said in a statement. "He plays Gold Glove-caliber defense and is an offensive force. In addition, he is a perfect fit for our goal of building for 2005 and beyond."
On Wednesday, 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.
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.
He 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.
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.
He 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.