Police said they were called to Pitt Lane in Bideford just after 06:00 GMT on Christmas Eve.
The 64-year-old victim was taken to Derriford Hospital in Plymouth with serious head injuries. He died on Boxing Day.
Officers said they do not know how the man was injured. His death is currently being treated as unexplained.
Officers want to hear from anyone who was in Pitt Lane in the early hours of Christmas Eve.
A man discovered unconscious in the middle of a road has died in hospital.