Oude IJssel (Dutch), literally old IJssel, or Issel (German) is a river in Germany and the Netherlands approximately 80 km long. It is a right tributary of the river IJssel. Oude IJssel means "Old IJssel" in Dutch; the Oude IJssel was the upper course of the IJssel until the connection with the Rhine was dug, possibly in the Roman era.
Since that moment the Rhine became the largest contributor to the flow of the IJssel, although only a relatively small amount of the total Rhine flow made its way into the IJssel system. Various tributaries can sometimes add quite some water to the total flow of the river, for example the Berkel and the Schipbeek. The IJssel river is the only branch of the Rhine delta that consumes tributary rivers instead of giving birth to distributary rivers. The latter only happens at the very last part of the river, where the small IJssel Delta is created.