Both vessels were seized in the coastal province’s Prasae Bay.
The first one seized was a small transport vessel that had been modified to carry fuel.
It was carrying an estimated 200,000 litres of illegal diesel fuel, officers said.
The second was a wooden fishing boat. It was carrying an estimated 100,000 litres of illegal diesel fuel, officers said.
They added that 11 crew members on the two vessels had been arrested.
Officials said they were able to detect the smugglers based on information obtained after a previous seizure of vessels smuggling fuel.
On June 17, authorities in the Third Naval Area Command seized two ships carrying an estimated 290,000 barrels of smuggled oil off Sichang Island in Chonburi province’s Si Racha district.
Thirteen crewmembers from the two ships were arrested. They provided authorities with information about smuggling that led to the seizure of the two ships off Rayong province on Saturday.