The full-grown female rhino was found dead at Borbhog camp in Burapahar range this afternoon with its horn removed, they said.
Empty cartridges of AK-47 and .303 rifles along with ammunition of double barrel gun were also found at the spot by the forest guards.
The horn can fetch a price between Rs 85 lakh and Rs one crore in the international market.
This is the third incident of a rhino being killed in the Burapahar range within the last one and a half months.
A massive combing operation has been launched in the area to nab the poachers, the sources said.
Altogether 25 rhinos have been killed by poachers in the national park this year while 70 rhinos have died, which include 23 deaths in flood and the rest due to natural causes.