Discharges from combined sewer overflows (CSOs) can effect hydraulic stress and significant impact on receiving waters. Common methods for monitoring CSO events like flow rate or water level are expensive in purchase, installation, operation and maintenance. Alternatively, cheaper surrogate methods can be used. This paper presents an experimental setup and a robust algorithm to detect CSO events by using low-cost temperature sensors. The convergence of simultaneous measurement signals from two temperature sensors installed in a CSO structure is used to detect CSO event frequency and duration. The first temperature sensor has to be installed directly in the sewer flow inside the CSO chamber to continuously measure the wastewater temperature. The second temperature sensor has to be installed on the crest of the overflow weir or on the invert of the CSO overflow channel to alternatively measure the temperature of the air during dry weather conditions and the temperature of spilled wastewater during a CSO event. The method was tested on a case study and the results were compared to common CSO detection by flow measurements as a reference. All 20 occurred CSOs in the investigation period of seven months could be detected without false detections and with an accuracy of 1 min.