During routine patrolling the forest guards found the body on a highland in Burha Pahar Range near Chirang Camp of the park adjacent to Phuloguri in Nagaon district.
Forest officials said the rhino had died due to old age.
This year 11 rhinos, including the two recently in the floods, have died due to natural causes in the World Heritage Site home to the Great Indian Rhinoceros, the officials said.
Last year there were 16 natural deaths of rhinos, including those killed by floods, they added.