The Capitals announced Wednesday that Alzner will earn $2.8 million per year from 2013-14 through the 2016-17 season.
The 24-year-old Alzner hasn't missed a game in three seasons. He had one goal and four assists last season and ranked third on the team in ice time per game and second in blocked shots.
Alzner and defenseman John Carlson are the only two players to appear in every Capitals game since the start of the 2010-11 season.
Alzner was a first-round draft choice by the Capitals in 2007. He has five goals and 39 assists in 263 career NHL games.