Four people were reported injured in the blast, which reportedly occurred during welding maintenance work on the tanker.
The blast happened at 9am as the tanker was moored at the quay for maintenance in Laem Yai subdistrict, at the mouth of the Mae Klong River.
Samut Songkhram's governor Somnuk Promkiew said the tanker was loaded with fuel, both oil and diesel.
Officials rushed to the scene to investigate the cause of the incident.
The explosion caused a fire that sent a plume of thick, black smoke billowing over the Samut Songkhram skyline, 80 kilometres southwest of Bangkok.
The cause of the explosion was still unknown as of press time.
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