Carlson, 22, is going into this third full NHL season. He scored nine goals and had 23 assists last season. Carlson set a team record for points by a rookie defenseman (37) the previous season.
In 186 NHL games, Carlson has 17 goals and 58 assists.
"John is a talented young defenseman who has played very well for the Capitals and still has room to grow," Capitals General Manager George McPhee said.
"He wanted to make a long-term commitment to the team and we are delighted that this young man who is an important part of our future will be a member of the club for many years to come."
Carlson will make $3.8 million this coming season and $4 million per season after that through 2017-18.