Wilder, 37-0 with 36 knockouts, was to have defended his World Boxing Council crown against Poland's Andrzej Wawrzyk.
Wilder, 37-0 with 36 knockouts, was to have defended his World Boxing Council crown against Poland's Andrzej Wawrzyk, 33-1 with 19 knockouts, but the European fighter tested positive for a banned substance.
Washington is 18-0 with one drawn and has fought mainly in southern California, his longest journey leading to a knockout of Ray Austin on the undercard of a Wilder title bout last July in Birmingham.
Wilder will be fighting before a home region crowd just as he did when he stopped fellow American Chris Arreola in the ninth round six months ago.
Wilder, 31, suffered a torn right biceps muscle and broken right hand in the victory and needed several surgeries and rehabilitation before being ready to step into the ring again.
"I was disappointed when Wawrzyk tested positive for a banned substance and wasn't available to fight," Wilder said. "I'm glad Gerald Washington stepped up to the challenge and allowed us to go forward with the show."
Washington, 34, is a US Navy veteran who played American football at the University of Southern California and on the practice squads of the NFL Seattle Seahawks and Buffalo Bills.
"I'm very happy to get this opportunity to fight for my first world championship," Washington said. "I know I'll be fighting Deontay Wilder in his backyard, but that adds more excitement to the fight for me."
Boxing's other major titles will be up for grabs on April 29 at London's Wembley Stadium when Britain's Anthony Joshua faces 40-year-old Ukranian former champion Wladimir Klitschko.