Cultural or tool use behaviours are typically conducted in social or food procurement contexts where the individual interacts with conspecifics, heterospecifics or environmental features. We report on postcoital penis cleaning in chimpanzees, an activity that does not fit this pattern. In penis cleaning, leaves are employed as ‘napkins’ to wipe clean the penis after sex. Alternatively, the same cleaning motion can be done without leaves, simply using the fingers. Not all chimpanzee communities studied across Africa clean their penes and, where documented, the behaviour is rare. By contrast, we identify postcoital penis cleaning in Budongo Forest, Uganda, as customary and corroborate penis cleaning as another cultural trait in chimpanzees, one that is specific to only a subset of the eastern subspecies of chimpanzee (Pan troglodytes schweinfurthii).