Zimmermann will earn $7.5 million next season and $16.5 million in 2015.
The 27-year-old right hander has been dominant since returning from Tommy John surgery in 2009. He's pitched over 600 innings since, going 40-30 for the Nationals with a 3.22 ERA and a 1.14 WHIP. He was named an All-Star for the first time last season, and finished seventh in NL Cy Young voting.
While the club would likely prefer a longer extension with one of their most productive pitchers, the two-year deal will allow them to avoid the arbitration process with Zimmermann this offseason and next winter while they weigh their options for a more substantial commitment.
Washington also avoided arbitration with reliever Drew Storen on a one-year contract worth $3.45 million. Storen, 26, can earn an additional $1 million in performance-based incentives in the deal. The right hander saved 43 games for the Nationals in 2011, but has been relegated to set-up and middle-relief duties due to injuries and the team's addition of Rafael Soriano last winter.