Sai Yok is a district (amphoe) in Kanchanaburi Province in western Thailand. The district is in the valley of the Khwae Noi River in the Tenasserim Hills area. It borders Myanmar to the south. Along the river the Death Railway runs to Nam Tok Sai Yok Noi. Within the district is Sai Yok National Park, created on 27 October 1980 covering an area of about 500 km2. The most famous sight in the park is the Sai Yok waterfall. Neighbouring districts are (from north clockwise) Thong Pha Phum, Si Sawat, Mueang Kanchanaburi of Kanchanaburi Province and Tanintharyi Division of Myanmar.
Apart from the nature of the Kwae Noi River valley, the other main attraction is Mueang Sing Historical Park where the westernmost Khmer-style temple complex is located.
Sai Yok is divided into seven sub-districts (tambons), which are further subdivided into 57 administrative villages (mubans).

Source: Wikipedia