Abstract: A rain overflow basin for collecting and storing rainwater, mixed water, or wastewater, has an inflow, an outflow, a basin chamber, a basin spillway, which is equipped with a weir threshold and is connected downstream of the basin chamber. A drain discharges the basin spillway, and a purification device purifies water flowing from the basin chamber to the basin spillway. A retention wall between the basin chamber and the basin spillway dams up water accumulating in the basin chamber. Water draining from the basin chamber is dammed up in a storage space is between the retention wall and the weir threshold, to reach the basin spillway via the weir threshold. The retention wall includes a passage opening. A purification device extends into the basin chamber such that water draining from the basin chamber through the passage opening into the storage space flows through the retention element.