In return, the Pirates will get young pitchers Wil Crowe, who made his debut with the Nationals in 2020, and Eddy Yean.
"We are thrilled to be adding a player of Josh Bell's caliber to our team," Nationals general manager Mike Rizzo said in a statement. "He adds a switch-hitting power bat to our lineup. And we know Josh will be an outstanding addition to our clubhouse and our community."
The 28-year-old Bell was an All-Star in 2019 following a torrid first half in which he hit .302 with 27 home runs and 84 RBIs. That form has been elusive over the past year-plus, however, as he hit .233 with 10 home runs during an injury-shortened second half of 2019 and hit just eight homers while batting a career-low .226 in 57 games for the Pirates in 2020.
Pittsburgh finished 19-41 last season, the worst record in the majors.
Bell believed he could be the cornerstone for Pittsburgh as the Pirates looked to retool under general manager Ben Cherington and manager Derek Shelton, both of whom came aboard following the team's disastrous second-half collapse in 2019 that led to a top-to-bottom revamp in the organization.
Instead, Bell will join the Nationals, just 14 months removed from a World Series title. He is under contractual control through at least the 2022 season.
"In order to build that winning team and get to that winning team, it's going to require some difficult decisions at times," Cherington said. "This is one of those.
"I believe what Pirates fans want is a winning team more than they want to root for a single player, even one who is a really good guy and a good player. If we think about that, that's how we see it.''
His departure also removes the Pirates' second-highest salary from the books. Bell, who was scheduled to make $4.8 million in 2020 before taking a fraction of that due to the truncated season, figured to receive at least a minor bump in arbitration this time around.
"We're going to need to continue to focus on the accumulation of talent and then the development of that talent, and that comes from all different avenues,'' Cherington said. "But trade is one of them, and this is the time of the year where those conversations happen, so we'll continue them.''
Rizzo had said adding a middle-of-the-order bat was his top offseason priority.
The 26-year-old Crowe was one of Washington's top pitching prospects. He went 0-2 with an 11.88 ERA in three starts for the Nationals in 2020.
Yean, 19, signed with Washington as a nondrafted free agent in 2020. He has posted a 3-5 record with 75 strikeouts while playing in the low minors.
The Associated Press contributed to this report.