The 28-year-old was set to be the top blue-liner available if he reached unrestricted free agency this offseason. Carlson's agent, Rick Curran, said earlier Sunday that he was taking calls from other teams.
The new deal puts Carlson in a tie with San Jose Sharks rearguard Brent Burns for the second-highest average annual value among NHL blue-liners, and just $1 million short of P.K. Subban's $9-million rate per season with the Nashville Predators.
Meanwhile, Oliver Ekman-Larsson reportedly has a verbal agreement on an eight-year extension with the Arizona Coyotes that will pay him $8.25 million per season beginning in 2019-2020, as 98.7 Arizona Sports' Craig Morgan first reported in May.
Carlson posted career highs in goals (15), assists (53), and points (68) during the 2017-18 regular season, and added 20 points in 24 playoff games en route to winning the championship with Washington.
He's spent his entire nine-year career with the Capitals, who selected him 27th overall in 2008.