Bryophytes have an important biogeochemical role in many forest ecosystems, where they regulate soil temperature and moisture and influence carbon and nitrogen (N) cycling. Associations between bryophytes and N₂-fixing cyanobacteria have been reported in northern latitude forests and are shown to provide a substantial N input to these ecosystems, but whether the association extends into temperate forests remains largely unknown. We investigated the extent of bryophyte-cyanobacteria associations in common understory bryophytes in a temperate forest in New York using δ¹⁵N analyses and ultraviolet fluorescence microscopy. Of the seven taxa examined, three (Fissidens taxifolius, Marchantia polymorpha, and Thuidium delicatulum) were associated with cyanobacteria, with colonization observed in 15–85% of the leaves examined. The taxa with cyanobacterial associations exhibited δ¹⁵N values that were 3.9‰ higher compared with taxa that did not, and the δ¹⁵N signatures were close to the atmospheric N₂ signature of zero, suggesting these taxa acquired a measurable fraction of their N from associated cyanobacteria. Taxa that associated with cyanobacteria also had 55% higher tissue %N that did those not associated with cyanobacteria. This study revealed the utility of natural abundance stable N isotope analysis in detecting bryophyte-cyanobacteria symbioses, and also suggests that bryophyte-cyanobacteria associations may represent a significant yet previously unaccounted for N source to the temperate forest biome.