Synthetic cathinones are among the most consumed new psychoactive substances (NPS), but their increasing number and interchangeable market make it difficult to estimate the real size of their consumption. Wastewater-based epidemiology (WBE) through the analysis of metabolic residues of these substances in urban wastewater can provide this information. This study applied WBE for the first time to investigate the presence of 17 synthetic cathinones in four European countries. A method based on solid-phase extraction and liquid chromatography coupled to tandem mass spectrometry was developed, validated, and used to quantify the target analytes. Seven substances were found, with mephedrone and methcathinone being the most frequently detected and none of the analytes being found in Norway. Population-normalized loads were used to evaluate the pattern of use, which indicated a higher consumption in the U.K., followed by Spain and Italy, in line with the European prevalence data from population surveys. In the U.K., where an entire week was investigated, an increase of the loads was found during the weekend, indicating a preferential use in recreational contexts. This study demonstrated that WBE can be a useful additional tool to monitor the use of NPS in a population.