The animal was discovered stranded and weakened at around 2 pm on May 3 near Vinh Hoi Hamlet in Cat Tien Commune, with multiple injuries on its back and around its eyes.
Dozens of fishermen attempted to return it to the sea, but by midday on May 4 it was found dead several hundred metres off Vinh Hoi Beach.
Residents later brought the carcass ashore for burial in accordance with local coastal fishing customs.
Le Van Toan, head of Vinh Hoi Hamlet, said the animal measured about four metres in length and was estimated to weigh more than two tonnes.
Residents described it as having a whale-like body, a pointed head and a long dolphin-like snout, leading many experienced fishermen to believe it may have been a beaked whale.
Authorities have launched an assessment to determine the species and cause of death.