On March 14, 2019, Sipp signed a one-year, $1 million deal with the Washington Nationals. The deal included a mutual option for the 2020 season. On June 7, Sipp hit 10 years of major league service time. He was designated for assignment on August 2 after posting an ERA of 4.71 in 36 appearances. He was released shortly afterwards. Reports involving the Houston Astros sign stealing scandal state that prior to the 2019 World Series Nationals ace Max Scherzer reached out to Sipp and asked if the Nationals needed to be concerned about the Astros even with no runners on base, to which Sipp replied yes.