Sources indicated on Saturday that Seattle has targeted the 30-year-old Guillen, who appeared in just 69 games last season before having surgery to reconstruct his right elbow in July.
ESPN.com reported Saturday that the Mariners have reached a one-year, $5 million contract with Guillen. The report said the contract includes the possibility of another $3 million in performance-based bonuses and has a $9 million option for the 2008 season.
Guillen hit .216 in 2006 with nine home runs and 40 RBIs before he had season-ending surgery.
Guillen has been taking batting practice since October and has indicated that he will be ready for the start of Spring Training in mid-February.
Guillen, 30, was offered arbitration on Friday by the Nationals, meaning he has until Dec. 7 to accept their offer.
When healthy, Guillen provides power from the right side of the plate. He hit more than 20 home runs in three consecutive seasons from 2003-05.
Should he join the Mariners, Guillen would likely serve as the team's designated hitter against left-handed pitching. Eduardo Perez held that role during the second half of last season, but wasn't offered arbitration on Friday.
Depending on the strength of his elbow, Guillen could also spend time in right field.
Guillen is a career .272 hitter with 143 home runs. In addition to the Nationals, he has played for Pittsburgh, Tampa Bay, Cincinnati, Arizona, Oakland and the Los Angeles Angels of Anaheim.
Guillen's best season came with Los Angeles in 2004 when he hit .294 with 27 home runs and 104 RBIs. He followed that with 24 home runs with Washington in 2005, despite being plagued by left shoulder problems.
But Guillen has made headlines for more than his bat during his Major League career.
In 2004, he was suspended by the Angels for the last two weeks of the regular season and the postseason for "inappropriate conduct" after publicly expressing his displeasure after manager Mike Scioscia removed him from a game.
Guillen was traded to the Nationals after the season.
Corey Brock is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.