The Capitals announced the deal Monday as they prepared for their first-round playoff series against the New York Rangers.
"He's like fine wine. He keeps getting better," coach Bruce Boudreau said. "The last half of the year he's been arguably our best player."
Knuble looked his age early in the season, when he had only one goal and three assists over the first 13 games. But he finished strong - with nine goals and four assists in his last 14 games.
He finished the regular season with 24 goals and 16 assists and missed only three games. The 24 goals ranked third in the NHL for players 35 and older, trailing only Martin St. Louis and Teemu Selanne, who both had 31.
Knuble has a stated goal of playing in 1,000 NHL games. He's at 968 over 14 seasons, so he should get there next year as long as he stays healthy and productive.
"The longer I delay myself to getting to the real world, the better," he said.