The Nan River is a major river in Northern Thailand and is a major tributary of the Chao Phraya River, Thailand’s largest navigable waterway. The Nan River originates in the Luang Prabang Range, Nan Province. The provinces along the river after Nan Province are Uttaradit, Phitsanulok and Phichit. The Yom River joins the Nan River at Chum Saeng District, Nakhon Sawan Province. When the Nan River joins with the Ping River at Pak Nam Pho within the town of Nakhon Sawan it becomes the Chao Phraya River. The Nan river runs about 630 kilometres (390 mi) north/south. The chief tributary of the Nan is the Yom River, which joins the Nan within Chum Saeng District in Nakhon Sawan Province. Other direct tributaries include the Butsabong and Wang Pong rivers of the lower Nan Basin, the Wat Ta Yom and Wang Thong Rivers which join the Nan within Phichit Province, the Khwae Noi River which joins the Nan within Phitsanulok Province, the Tron and Pat Rivers, which join the Nan within Uttaradit Province, the Wa, Haet, Pua, Yao and Hao Rivers, which join the Nan within Nan Province. The only significant waterfall on the Nan River is the Kaeng Luang. Between 1976 and 1985 the Naresuan Dam was constructed north of the city of Phitsanulok as part of the Phitsanulok Irrigation Project.

The total area drained by the Nan and its tributaries is 57,947 square kilometres (22,373 sq mi). Much of the lower basin has perfect soil for farming.