Ko Sai (Thai: เกาะทราย, pronounced [kɔ̀ʔ sāːj]) is one of the gulf islands of Thailand.

It lies on the west side of the Gulf of Thailand, about 2 km off-shore.

[1] Ko Sai is the largest of a group of three rocky islets also known as "Pran Rocks", off the fishing village of Khao Takiab, just south of Hua Hin.

This area belongs administratively to Prachuap Khiri Khan Province.

Local tour operators call it "Lion Island" (Ko Singto) and organize regular sightseeing and snorkeling excursions to visit it.