Kwai Yai River , also known as the Si Sawat, is a river in western Thailand. It flows for about 380 kilometres through Sangkhla Buri, Si Sawat, and Mueang Districts of Kanchanaburi Province, where it merges with the Khwae Noi to form the Mae Klong River at Pak Phraek subdistrict.

The famous bridge of the Burma Railway crosses the river at Tha Makham Subdistrict of the Mueang District. The construction of the bridge is depicted in The Bridge over the River Kwai by Pierre Boulle and in its film adapttion.

Up until the 1960s, the River was considered part of the Mae Klong itself.