About 50 tonnes of crude oil spilled into the sea off Rayong on Saturday morning after a leak occurred in a pipeline operated by PTT Global Chemical Plc (PTTGC).
PTTGC said the oil slick, enough to fill one tanker truck, was quickly contained.
The incident occurred at 6.50am about 20 kilometres southeast of Map Ta Phut seaport in the eastern province.
The company said it detected the leak as crude from an Omani tanker moored offshore was being transferred to the pipeline for delivery to the PTTGC refinery at Map Ta Phut.
The amount of crude that spilled into the sea was about 50 tonnes or equivalent to the capacity of one large tanker truck, said PTTGC.
As soon as it detected the leak, the company said workers shut the pipeline valve to prevent any further spills.
Anon Sirisaengtaksin, the chief executive of PTTGC, said quick action by the company in cooperation with the Royal Thai Navy and the Marine Department contained the damage.
"All the crude had been eradicated an hour after the accident and residents and fishermen near Map Ta Phut will not be affected by the crude oil spill," said Mr Anon.
He said the company staged frequent accident drills to deal with spills and related incidents. It also monitors pollution with the Industrial Estate Authority of Thailand (IEAT) and the Pollution Control Department.
Mr Anon said PTTGC would continue to monitor conditions in the Gulf of Thailand until it can be certain that not a drop of crude oil remains in the water.
"Tourists and fishermen who live there can be sure that no oil spill will reach their places," said Mr Anon.