Sipp's deal is for one year and $1 million with a mutual option for 2020 that includes a $250,000 buyout, according to ESPN's Buster Olney.
Nationals' projected bullpen - 2018 stats
The 35-year-old Sipp spent the past five seasons with the Houston Astros.
In 2018, he allowed 0.2 HR/9, the lowest mark of his career, while his 83.3 LOB percentage was the second-best result of an Astros reliever with at least 30 innings of work.
Over the course of his 10-year career, Sipp owns a 3.67 ERA and has struck out 9.6 hitters per nine innings while pitching for the Astros, Cleveland Indians, and Arizona Diamondbacks.
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