This paper provides information on butterflies of the lowland forests of Bhutan for the first time. As a part of the biodiversity impact assessment for the proposed Sankosh hydroelectric power project, a survey was carried out along the Sankosh River catchment to study the butterfly diversity. The aim of the study was to identify species of conservation priority, their seasonality and to know the butterfly diversity potential of the area. Surveys were carried out during five different seasons (winter, spring, pre-monsoon, monsoon, post-monsoon) lasting 18 days from January 2009 to March 2010. Pollard walk method was used to assess the diversity on four-line transects within 10-12 km radius of the proposed dam site. Two hundred and thirteen species, including 22 papilionids, were thus sampled. Eleven species amongst these are listed in Schedules I and II of the Indian Wildlife (Protection) Act, 1972, of which 10 taxa (Pareronia avatar avatar, Nacaduba pactolus continentalis, Porostas aluta coelestis, Elymnias vasudeva vasudeva, Mycalesis mestra retus, Melanitis zitenius zitenius, Charaxes marmax, Athyma ranga ranga, Neptis manasa manasa and Neptis soma soma) are of conservation priority as they are â€˜rareâ€™ in occurrence across their distribution range in the region. The maximum number of species (128) were recorded during the spring season (March) and lowest (66) during July (monsoon). The seasonal pattern of variation in diversity was very typical of the pattern found in other areas of the lower foothills and adjoining plains of the Himalaya. Relative abundances of butterflies during spring varied significantly (p<0.05) as compared to winter, pre-monsoon and post-monsoon seasons. However, species composition changed with every season as Sorensenâ€™s similarity index varied between 0.3076 to 0.5656. All these findings suggest that the lowland forests of Bhutan hold a rich and unique diversity of butterflies during every season of the year thus having a good potential for ecotourism. Establishment of a butterfly conservatory and park as a â€˜biodiversity offsetâ€™ for conservation of â€˜rareâ€™ species along with more field surveys in the study area will be a way forward along with the present work to bridge the exisiting gaps in knowledge on butterflies of the sub-tropical lowland forests of the Himalayas.