The man climbed the 20-foot (six-metre) wall of Chhatbir Zoo, home to four lions, on Sunday and entered the restricted area where he was attacked.
Hearing his screams, the staff rushed to try and rescue the man.
"He was an intruder in the zoo. We took him to the hospital but he succumbed to his injuries," said Roshan Sunkaria from the state forest department.
The animal that attacked the man was an Asiatic lion -- a critically endangered species and a major tourist draw.
Only around 500 exist in the wild, all in the Gir sanctuary in the western Indian state of Gujarat.
The authorities have not yet been able to contact the victim's family.
The zoo has stepped up warnings inside the premises and advised visitors to travel with an escort and keep vehicles locked.