Chara, 43, will make $795,000 against the salary cap with Washington. He said the Bruins decided to go in a different direction, and the unrestricted free agent was not in their plans.
"The Boston Bruins informed me that they plan to move forward with their many younger and talented players and I respect their decision," Chara wrote in a statement posted to Instagram.
Chara, the Bruins' captain, played 1,023 games with Boston after arriving as a free agent in July 2006. He had 148 goals and 333 assists and was a plus-240 for the Bruins, acting as the linchpin for one of the most dominant defensive teams in the league over the past decade. He captained three Bruins teams to the Stanley Cup Final, winning the championship in 2011.
The 6-foot-9 defenseman was a five-time finalist for the Norris Trophy with the Bruins, winning the award once in 2008-09.
"Unfortunately, my time as the proud Captain of the Bruins has come to an end. I want to first of all thank the passionate and loyal Bruins fans, who shared the ups and downs of each season over the past 14 years. I'm proud that we were able to return the Stanley Cup to Boston and celebrating with all of you, in Boston, New England, and around the world, was a moment I will never forget," he wrote.
"As I begin this next chapter, I want the people of Boston to know how proud I was to be a Bruin and how grateful I am for all of the support over the years. 'Thank you' does not seem adequate to express my sincere gratitude. I will always be a Bruin. I will always love Boston."
Chara joins a Capitals team that has lost in the first round of the playoffs in two straight seasons after winning the Stanley Cup for the first time in 2018. The Capitals hired coach Peter Laviolette in an effort to spark their veteran team; adding Chara, one of the NHL's most feared and respected captains, bolsters that effort.
"We are extremely pleased to have Zdeno join the Capitals organization," said senior vice president and general manager Brian MacLellan. "We feel his experience and leadership will strengthen our blue line and our team."