The River Idle is a river in Nottinghamshire, England. Its source is the confluence of the River Maun and River Meden, near Markham Moor. From there, it flows north through Retford and Bawtry before entering the River Trent near Misterton.

Most of the land surrounding the river is a broad flood plain, which is now partly occupied by a number of sand and gravel pits. Its main tributaries are the River Poulter and the River Ryton.

In the year 616 or 617, the East Angles under Raedwald defeated the Northumbrians under Aethelfrith here, and Aethelfrith was killed. The battle resulted in the establishment of Edwin as king of Northumbria.