The deal does not extend the length that the Nationals control Zimmermann, but it effectively buys out Zimmermann's final two years of arbitration and provides him a set salary through the 2015 season.
Zimmermann will make $7.5 million in 2014 and $16.5 million in 2015, the source confirmed.
The New York Post was first to report the Nats and Zimmermann reaching terms on the two-year deal.
In addition, the Nats also reached terms with right-handed reliever Drew Storen on a one-year deal, avoiding arbitration. Per the New York Post, Storen will make $3.45 million in 2014 with incentives that could pay him another $1 million in bonuses for games finished.
Zimmermann was the Nationals' most consistent starter in 2013, going 19-9 with a 3.25 ERA.
He made $5.35 million last season, and while the Nationals still would like to lock him up to a long-term deal, this two-year contract at least allows them to avoid the arbitration process this year as well as next year, if they're unable to finalize a new deal.
Storen went 4-2 with a 4.52 ERA during a rocky 2013 season that saw him get demoted to Triple-A Syracuse after a rough stretch in July.
The right-hander, who saved 43 games for the Nats in 2011, finished the season strong, however, posting a 1.40 ERA over his final 21 big league apperances.
Storen gets a raise over the $2.5 million he made last season.
The Nats now have five arbitration-eligible players left without contracts at this point: Jerry Blevins, Tyler Clippard, Ian Desmond, Doug Fister and Wilson Ramos.