The horn of the female rhino was missing, a park official said. The poaching took place in the Burhapahar range of the park.
The official said forest guards exchanged fire with the poachers but they managed to escape.
A total of 13 one-horned rhinos have been killed in Kaziranga, a UNESCO world heritage site, by poachers so far this year, according to official statistics. Poachers also killed one rhino each in Manas National Park and Pabitora Wildlife Sanctuary this year.
Last year, poachers killed over 40 rhinos in Assam.
The Assam government has set up an elite unit - Assam Forest Protection Force - and equipped it with AK series rifles to fight poachers.