The 33-year-old cited his opposition to Trump's administration and "divisive rhetoric" as the reason behind his decision.
"At the end of the day, as much as I wanted to be there with my teammates and share that experience with my teammates, I can't do it," Doolittle told Jesse Dougherty of the Washington Post. "I just can't do it."
"People say you should go because it's about respecting the office of the president," he continued. "And I think over the course of his time in office (Trump's) done a lot of things that maybe don't respect the office."
Doolittle added that his teammates all respect his choice to skip the ceremony, and he will do the same for those who decide to attend. Thus far, he's the only member of the team who has publicly declined Trump's invitation.
Championship teams visiting the White House has long been a tradition in North American pro sports but has grown in controversy during the Trump era. Several members of the 2018 World Series champion Boston Red Sox, including manager Alex Cora, skipped their visit to 1600 Pennsylvania Avenue.
The Nationals' victory parade will take place in Washington on Saturday afternoon.