The River Chelmer is a river that flows entirely through the county of Essex, England.

The source of the river is near Debden Green, a village near Thaxted. The source of the River Can is also nearby. The River Chelmer flows past Thaxted, south through the district of Uttlesford around the northeast of Great Dunmow. It continues flowing south-southeast into the borough of Chelmsford and on into the town of Chelmsford where the River Can flows into it. It then flows east through the borough and into the district of Maldon until it meets the River Blackwater near Maldon. It discharges into the North Sea via the Blackwater Estuary

The Chelmer and Blackwater Navigation Company was found by act of parliament in 1793. Work then commenced on constructing the navigation from Chelmsford to Colliers Reach in the tidal estuary of the River Blackwater. The work was completed in 1797. The navigation mainly followed the course of the River Chelmer from Chelmsford to Beeleigh near Maldon. From there it continued through a short cut and then followed the course of the River Blackwater to Heybridge.