The Tapi (or Tapee) river () is the longest river in southern Thailand. The river originates at the Khao Luang, and has a wide estuary into the Gulf of Thailand at Bandon Bay near the town of Surat Thani. It has a length of 230 km.

The river drains an area of 5460 km² and in 1997 had an annual discharge of 135.4 m³/s or 4.3 km³ per year. The Phum Duang River (or Khirirat River), which drains another 6,125 km² west of the Tapi watershed, joins the estuary 15 km west of Surat Thani in Amphoe Phunphin.

The river was named in August 1915 after the river Tapi in India, shortly after the town of Surat Thani was named after the town Surat.

The island of Ko Lamphu (เกาะลำพู) is located in the Tapi River, about 9 Km from its mouth, right by the Surat Thani town center.

In 1975 an area of 29.6 km² of swamp land on the east bank of the river in the district Khian Sa was declared the Nong Thung Thong non-hunting area.