The Nam Nao National Park was alerted to the dead elephant near the road to Ban Huay Rahong village.
Park officials and a veterinarian found that it was about 10 to 15 years old and weighed about two tonnes. The vet believed it died about three to four days earlier.
A preliminary post-mortem said the elephant was poisoned because its tongue and liver were black. Samples from its stomach and food were sent for lab tests.
The elephant was buried.