While on rehab assignment with the Triple-A Nashville Sounds, Doolittle pitched the seventh inning of a combined no-hitter against the Omaha Storm Chasers on June 7, 2017. Starter Chris Smith pitched the first six innings and was then followed by Doolittle, Tucker Healy, and Simón Castro who each pitched one inning.
On July 16, 2017, Doolittle was traded to the Washington Nationals, along with Ryan Madson, for Blake Treinen, Sheldon Neuse, and Jesus Luzardo. On July 18, Doolittle recorded his first save for the Nationals in a 4-3 win over the Los Angeles Angels. In 30 games for the Nationals, he was 1-0 with a 2.40 ERA in 30.0 innings and was 21/22 in save opportunities.
Doolittle and Eireann Dolan married on October 2, 2017, eloping the day after the Washington Nationals' last game of the regular season.