The River Maun is a river in Nottinghamshire, England. Its source lies in Kirkby-in-Ashfield, and from there it flows north east through Mansfield (which takes its name from the river), Edwinstowe and Ollerton, these being the heart of the Sherwood Forest area. It becomes known as Whitewater near the village of Walesby and connects to the River Meden temporarily where the Robin Hood Way crosses them. They diverge, and near Markham Moor it merges again with the River Meden this time becoming the River Idle. Its main tributaries are Rainworth Water, Vicar Water and Cauldwell Water.
The river has been an important source of power, from at least 1086, when there was a watermill in Mansfield. A big increase in the number of mills began in the 1780s, when the frame knitting industry was decimated by the advent of Richard Arkwright's water-powered spinning frame. William Cavendish-Bentinck, 3rd Duke of Portland, encouraged the building of textile mills to relieve unemployment and poverty. Most were converted to do "cotton doubling", and several later became hosiery mills. The conversion of watermills which had formerly ground corn to textile mills led to the building of windmills to carry on milling corn. Although water power has largely ceased, there is still an operational water-mill at Ollerton.