The Panthers are receiving a fifth-round pick in return, according to Ian Rapoport of the NFL Network.
Allen made 12 starts last season filling in for an injured Cam Newton. After a hot start which saw him win his first four games, the young passer struggled over his next eight. He finished with a 5-7 record, accumulating 3,027 passing yards, 17 touchdowns, and 15 interceptions.
The 24-year-old was deemed expendable in Carolina after the Panthers signed Teddy Bridgewater last week and former XFL star P.J. Walker on Monday. The team's quarterback room also features 2019 third-rounder Will Grier. Longtime starter Cam Newton is expected to be traded or released in the coming weeks.
Allen reunites with head coach Ron Rivera in Washington. Rivera brought him in as an undrafted free agent in 2018 while with the Panthers, and he'll likely provide stability behind sophomore starter Dwayne Haskins in 2020.