Some primates regularly wash their hands and feet with urine. This behaviour is outwardly similar to scent marking in other mammals, but it differs in that the urine is applied to the bare skin of the hands and feet rather than rubbed into the fur or applied directly onto an object in the environment. Empirical evidence for the functional significance of urine-washing remains inconsistent. We used rigorous statistical methods to examine environmental and social influences on urine-washing behaviour, using 4380 observation hours on five groups of wild white-faced capuchins (Cebus capucinus) in Costa Rica. Urine-washing frequencies were most strongly affected by environmental dryness, both within and between seasons, with markedly less urine-washing during humid conditions. Increased individual activity levels also promoted urine-washing. Among females, urine-washing was less frequent during lactation than during pregnancy and other reproductive states. Among males, urine-washing frequencies were greater in alpha males, who also exhibited a ‘vigorous’ form of urine-washing that may be functionally distinct. During the dry season, 3∕5 groups exhibited more urine-washing than expected near fruit trees, but across groups there were no consistent spatial patterns for urine-washing with respect to water resources, home range overlap zones, core areas, inter-group encounter zones, and the home-range periphery. Urine-washing appears to differ fundamentally from common forms of mammalian scent marking. We suggest that its function is primarily mechanical, perhaps to apply a sticky residue to the hands and feet to improve grip on dry, arboreal substrates. Lesser signalling functions may include sexual signalling and resource labelling.