Officials confirmed to ESPN.com on Tuesday that bout agreements have been signed. The UFC 200 pay-per-view event will take place at T-Mobile Arena.
Dillashaw (12-3), 30, had campaigned for an immediate championship rematch against Dominick Cruz, but the UFC opted to go another direction. Dillashaw, who trains out of Denver, lost a narrow split decision to Cruz in January.
Before the loss, Dillashaw had won four in a row, including a resounding TKO against Renan Barao for the 135-pound title in May 2014.
Assuncao (23-4), 33, hasn't fought since October 2014 because of an injury. He will look to pick up where he left off and add to a seven-fight winning streak.
The two met previously at a UFC Fight Night event in 2013 in Brazil. Assuncao defeated Dillashaw via split decision. Dillashaw has always believed judges robbed him of a win on the scorecards in that fight.
A light heavyweight title unification bout between Daniel Cormier (22-1) and interim champion Jon Jones (17-1) will headline UFC 200. Officials have not yet confirmed the Dillashaw-Assuncao rematch's placement on the card.