Tropical mountain cloud forests (TMCF) harbour a high bryophyte (mosses and liverworts) biomass and diversity. Furthermore, the high air humidity makes these forests well suited for bryophyte-associated nitrogen (N₂) fixation by cyanobacteria, providing a potentially important source of N input to the ecosystem. However, few studies have assessed bryophyte-associated N input in these ecosystems, and these have focused on epiphytic bryophytes, whereas abundant ground-covering bryophytes have not been included. In this study, we quantified N₂ fixation rates associated with bryophytes, focusing on ground-covering mosses in a neotropical mountain cloud forest. Furthermore, we identified the effects of climate change (higher temperature 10 vs. 20° and lower bryophyte moisture level 50% vs. 100%) on N₂ fixation across bryophyte species and groups (mosses and liverworts). Nitrogen fixation rates associated with ground-covering moss species were up to 2 kg N ha⁻¹ year⁻¹, which is comparable to other N inputs (e.g. N deposition) in tropical cloud forests. Furthermore, changes in temperature showed little effect on N₂ fixation, but low moisture levels significantly suppressed N₂ fixation activity. We found low N₂ fixation activity associated with the investigated liverworts. Our results demonstrate the importance of ground-covering, moss-associated N₂ fixation as a N source in tropical cloud forests and suggest that predicted future declines in precipitation in these systems will reduce N inputs from bryophyte-associated cyanobacteria. Read the free Plain Language Summary for this article on the Journal blog.