Authorities found the gaur while conducting a field trip in the forest to check numbers of the remaining gaur population in the park, Theerapat Prayurasiddhi, deputy director-general of the National Parks, Wildlife and Plants Conservation Department, said on Wednesday.
The animal is believed to have been dead for a month.
That means it could have died in the same period as the previous 19 gaur found dead at the national park.
A total of 20 gaur carcasses have now been found inside the park since early December, he said.
Authorities suspected they died from poisoning or a disease outbreak.
Mr Theerapat said the department had sent samples of soil, water and plants in the area for testing to find if there are any toxic substances that caused the gaurs' death. The National Institute for Animal Health has also been testing samples from the gaur carcasses.
He said authorities would meet on Friday to give updates on the investigation and results of the latest laboratory tests.