In most unisexual species, the numbers of male and female individuals are approximately equal. The adult sex ratio (ASR) is a key demographic parameter for the conservation and management of wild species. However, in the case of monomorphic birds such as the Rock Ptarmigan Lagopus muta, estimating the sex ratio is difficult. To test if the sex ratio is balanced, we analysed the adult sex ratio of this tetraonid with two different methods, using microsatellite individualisation of non-invasive samples (faeces) and the hunting bag data of birds hunted in the French Alps and Pyrenees. The results of our non-invasive study revealed a balanced ASR in spring, whereas hunted specimens had a strongly male-biased ratio for adults but an approximately balanced sex ratio for juveniles (< 1 year old). The sex ratio estimated from hunting data may be biased for the adult population and, as occurs in other species, is not representative of the overall sex ratio. Our results suggest that, assuming our sampling strategy was adequate, non-invasive sampling is a good method to estimate ASR.—Aleix-Mata, G., Pérez, J.M., Adrados, B., Boos, M., Marty, E., Mourieres, P., Lauer, E., Zimmermann, S., Mossoll-Torres, M. & Sánchez, A. (2021). Sex ratio of Alpine and Pyrenean Rock Ptarmigans Lagopus muta estimated by a non-invasive method (faecal DNA analysis) and from hunting bag data. Ardeola, 68: 201-212.