WBO light-middleweight champion Saul 'Canelo' Alvarez will fight fellow Mexican Julio Cesar Chavez Jr on 6 May.
The fight will take place at a catchweight between middle and super-middleweight - 164.5lbs - and so will not be for a world belt.
Alvarez, 26, had seemed set to fight WBC, WBA and IBF middleweight champion Gennady Golovkin, after saying he was "ready" for him in September.
That came after he won the WBO belt from Liverpool fighter Liam Smith.
Alvarez also beat Smith's compatriot Amir Khan in 2016, knocking the Bolton fighter out to defend the WBC middleweight title in May, before vacating the belt later that month.