Sai Yok (, ) is a district (amphoe) in Kanchanaburi province in western Thailand and is located northwest of Bangkok.
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.
Sai Yok National Park was created on 27 October 1980 and covers an area of about 500 km<sup>2</sup>. The most famous sight in the park is the Sai Yok waterfall.
Neighboring districts are (from north clockwise) Thong Pha Phum, Si Sawat, Mueang Kanchanaburi of Kanchanaburi Province and Tanintharyi Division of Myanmar.
Sai Yok is divided into seven subdistricts (tambons), which are further subdivided into 57 administrative villages (mubans).
As of December 2024 there are: three municipal (thesaban) areas in the district: Sai Yok subdistrict municipality (thesaban tambon) consists of the whole subdistrict Sai Yok, Nam Tok Sai Yok Noi subdistrict municipality () covers parts of Tha Sao subdistrict and Wang Pho subdistrict municipality () consists of parts of Lum Sum subdistrict.
The non-municipal areas are administered by six subdistrict administrative organizations - SAO (ongkan borihan suan tambon - o bo toh).
Sai Yok district is served by two hospitals:
In the district there are twelve health-promoting hospitals in total, of which;
There are sixty-eight Theravada Buddhist temples in the district.
Christians have their thirteen churches and muslims have one mosque.
Apart from the nature of the Kwae Noi River valley, the other main attraction is Mueang Sing Historical Park, which shows the westernmost Khmer-style temple complex.