The carcass of the rhino was found yesterday by patrolling forest guards in the sanctuary, located 38 km from Guwahati, they said.
The rhino was killed by the poachers about two days back.
Large parts of the forest have been inundated by flood waters, forcing most of the animals to move to higher grounds for shelter making them easy targets of poachers, they said.
The 38.8 square-km sanctuary has a dense population of the Great Indian One-Horned Rhinoceros found in its habitat which effectively spans to an area of 16 square km of the forest.