The death of Mr Beshta was reported to Koh Lipe police at about 5:30am, reports the Bangkok Post.
Capt Bidan Saripa of the Koh Lipe Police said Mr Beshta and his 17-year-old son, Ostap, arrived on the island on Friday (May 28) for a vacation. They stayed in the same room at Bunga Resort.
There were no traces of the room being forced open or of the ambassador being attacked, the officer said.
According to his son, Mr Beshta went to bed at 11pm Saturday night. At 4:30am Sunday, Mr Beshta began vomiting, lost consciousness and died shortly afterwards, his son said.
The body of the Ukrainian ambassador was later taken from Koh Lipe island to Satun Hospital. There, Satun Governor Ekarat Leesen was on hand to receive the body.
An arrangement was subsequently made to transport the ambassador’s body to Bangkok for an autopsy.
Although so far there have been no suggestions of foul play, given the longstanding hostility between Russia and Ukraine, speculation is inevitable.
Former Ukraine politician Viktor Yushchenko was disfigured in a 2004 dioxin poisoning linked to Russia. He went on to become president.
In 2018, the British government accused Russia of attempted murder and announced a series of punitive measures after the Novichok poisoning of former spy Sergei Skripal and his daughter Yulia.
Russia is also widely believed to be behind the Novichok poisoning of opposition leader Alexei Navalny on Aug 20 last year.
In 2014, Russia invaded and annexed the Ukrainian territory of Crimea, and this year fears have grown that Russia is planning a new invasion. The enmity between the neighbouring countries has grown into one of the major flashpoints in relations between Russia and the West, with tensions escalating in recent months.