The Haolai River () is a narrow natural river located on the Inner Mongolia Plateau in northern China. Often referred to as the "narrowest river in the world", it stretches approximately 17 kilometers in length. Its average width of 15 centimeters can be crossed with a single step. The river is just 4 centimeters wide at its narrowest point. While some skepticism exists regarding this claim, especially given its extreme narrowness, the river meets the accepted scientific criteria for classification as a river, rather than a stream or rill.
Locally, the river is also known as the "Book Bridge River", a name derived from local folklore. According to legend, a young boy once dropped a book while crossing the river, which inadvertently created a bridge for ants to cross.