The River Kwai, more correctly Kwai Noi (Thai: แควน้อย, English small tributary) or Kwai Sai Yok (แคว ไทรโยค), is a river in western Thailand, near, but not over the border with Myanmar. It begins at the confluence of Ranti, Songkalia and Bikhli Rivers. At Kanchanaburi it merges with the Kwai Yai River to form the Mae Klong river, which empties into the Gulf of Thailand at Samut Songkhram.
The river is chiefly known from the Pierre Boulle novel and David Lean film The Bridge on the River Kwai, in which Australian, Dutch and British prisoners of war were forced by the Japanese to construct two parallel bridges spanning the river as part of the Burma Railway, also called the Death Railway, for the many lives lost in its construction. One bridge was wooden and temporary. The other was made of concrete and steel and still exists. The bridges actually spanned the Mae Klong, but as the railway subsequently follows the Kwai Noi Valley, the bridges became famous under the wrong name. In the 1960s, the upper part of the Mae Klong was renamed the Kwai Yai ("big tributary").