Hatfield, 63, died in his sleep, the group's manager, David Cohen, said.
Police responded to a call at the Radisson Hotel in Kalamazoo at about 7 p.m. EST. Five minutes later, in Miller Auditorium -- where the Righteous Brothers were to begin their tour -- an announcement was made that the concert had been canceled because of a personal emergency.
Hatfield and Medley began performing together in 1962, singing in a style that would be dubbed "blue-eyed soul." The two were signed within a year to Moonglow records, which released their debut single, "Little Latin Lupe Lu."
The duo's breakthrough came in December 1964 with the release of their ballad "You've Lost That Lovin' Feelin'."
This March, Hatfield and Medley received the highest honor for a pop music artist, induction to the Rock and Roll Hall of Fame.