The deal is for $5 million with an opportunity to earn $1 million more in performance bonuses, a source told ESPN.
Candelario, 29, was non-tendered by Detroit earlier this month, making him a free agent after he compiled a career-low .633 OPS in 2022. He'll have a chance to rebuild his value in Washington, where the Nationals are in the early portion of a rebuilding phase.
Candelario is a seven-year veteran who led the majors in doubles (42) in 2021 but had just 28 walks to 109 strikeouts in 2022. He has a career .723 OPS in 606 games played.
This will be Candelario's third team. He came up with the Chicago Cubs in 2016 before being traded to Detroit the following season. He hit 19 home runs in 2018 but has yet to reach that total in subsequent years.
The Nationals on Tuesday also signed outfielder Stone Garrett, who hit. .276 with four home runs in 27 games for Arizona last season. Garrett will be under team control for six seasons.