The extension is for $5.75 million -- $3 million in 2011-12 and $2.75 million in 2012-13. The $14 million, four-year deal he signed when he joined Washington as a free agent in 2007 was set to expire after the coming season.
"Tom has played very well for the Washington Capitals and has been a key part of our success since he joined the team," vice president and general manager George McPhee said in a statement. "We are pleased that he has made the commitment to remain a Capital."
Last season, Poti was struck in the eye by a puck in Game 6 of the first round of the playoffs against Montreal.
Poti broke four bones in his face and lost sight in his right eye for more than 24 hours, but doctors at the time said they were "100 percent confident" he would make a full recovery. The Capitals were eliminated by the Canadiens in Game 7.
The 33-year-old Poti had four goals, 20 assists and 42 penalty minutes in 70 games last season, helping Washington finish with the NHL's best regular-season record.
He ranked fifth among league defensemen with a career-best plus-26 rating and ranked second on the Capitals with 121 blocked shots.
The Capitals also said Tuesday that they have signed free-agent goalie Brandon Anderson to a three-year, entry-level contract.
Information from ESPN.com's Pierre LeBrun and The Associated Press contributed to this story.