The Red Sox have exclusive negotiating rights with Varitek, who is 36, for the next 15 days. If that window expires without a deal, Varitek is eligible to sign with any of Major League Baseball's 30 teams.
Utility infielder Alex Cora and veteran backup catcher David Ross were the other Boston players who filed for free agency. Curt Schilling, Sean Casey, Mike Timlin, Paul Byrd and Mark Kotsay, the other Boston players eligible for free agency, are likely to do so over the next few days. Schilling and Timlin have both hinted at retirement.
Varitek was last a free agent following the 2004 season and wound up signing a four-year deal to stay with the Red Sox on Christmas Eve.
The switch-hitter suffered through the worst offensive season of his career in 2008, hitting .220 with 13 homers and 43 RBIs. But he remained a key factor on defense, lauded for his ability to handle the pitching staff and block the plate.
Red Sox general manager Theo Epstein expressed uncertainty about Varitek's future when asked about it the day after Boston's loss in Game 7 of the American League Championship Series to Tampa Bay.
"He's an important part of the organization -- there's no doubt about that," Epstein said. "Obviously he's coming off a year that wasn't his best, but he's important nonetheless. Now, he's a free agent and we'll be talking to him. We have an obligation to explore all our options. We do that with every position."
Cora has been a valuable member of the Boston bench since coming over in a trade from the Indians in July, 2005.
The left-handed-hitting veteran has enjoyed his time with the Red Sox and would like to return. Cora hit .270 in 75 games in 2008.
Ross was signed by the Red Sox to a Minor League contract on Aug. 23 to provide organizational depth. He was on Boston's postseason roster for both the Division Series and the ALCS, serving as the third catcher behind Varitek and Kevin Cash.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.