The River Kwai, more correctly Kwhae Noi (English small tributary), is a river in western Thailand, near the border with Myanmar. At Kanchanaburi it merges the Mae Klong river, which empties into the Gulf of Thailand in Samut Songkhram.

The river is chiefly known for the Pierre Boulle novel and David Lean film The Bridge on the River Kwai, in which British prisoners of war are forced by the Japanese to construct a bridge spanning the river to complete the Burma Railway. However the bridge actually spanned the Mae Klong, but as the railway subsequently follows the Kwae Noi valley the bridge became famous under a wrong name. In the 1960s the upper part of the Mae Klong was renamed to Kwae Yai (big tributary)