The River Medlock is a river of Greater Manchester in north west England that flows for 10 miles before joining the River Irwell in central Manchester.

Rising in the hills that surround Strine Dale just to the east of Oldham it flows through the steep-sided wooded gorge that separates Oldham from Ashton-under-Lyne and Daisy Nook Country Park with its 19th century aqueduct carrying the disused Hollinwood Canal over the shallow river. Along its course, the valley provides a welcome respite from the urban sprawl of the east Manchester suburbs, and is perennially popular with locals.

As it reaches the city centre it frequently disappears underground, passing underneath the UMIST campus and Oxford Road station before effectively becoming part of the Bridgewater Canal at the end of Castle Street, 400 metres before its original junction with the River Irwell.