The Khwae Yai River (Thai: แม่น้ำแควใหญ่, RTGS: Maenam Khwae Yai, IPA: [mɛ̂ːnáːm kʰwɛː jàj]), also known as the Si Sawat (แม่น้ำศรีสวัสดิ์ [mɛ̂ː náːm sǐː sa.wàt]), is a river in western Thailand.

It has its source in the Tenasserim Hills and flows for about 380 kilometres (240 mi) 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.

'[1] In 1980, the Srinagarind Dam (Thai: เขื่อนศรีนครินทร์; rtgs: Khuean Sinakharin) on the Khwae Yai was completed in Si Sawat District of Kanchanaburi Province.

The famous bridge of the Burma Railway crosses the river at Tha Makham Subdistrict of the Mueang District.

Up until the 1960s, the river was considered part of the Mae Klong itself, but this part of the Mae Klong was then renamed Khwae Yai to bring geographical fact more in line with the fictional association with the name River Kwai.