The Umzimkulu River is a river in South Africa. It rises in the Drakensberg mountains at the confluence of the Ngwangwane and the Underberg River. It flows southeast towards the Indian Ocean, which it enters through an estuary at Port Shepstone. Towns on the Umzimkulu include Underberg and Umzimkulu. Its main tributary is the Bisi River which joins its right bank about halfway down its course.In the past, the Umzimkulu formed part of the border between Eastern Cape and Kwazulu-Natal. Presently this river is part of the Mvoti to Umzimkulu Water Management Area.