Biological effects of ultraviolet radiation (UVR; 280–400 nm) on marine primary producers are of general concern, as oceanic carbon fixers that contribute to the marine biological CO<sub>2</sub> pump are being exposed to increasing UV irradiance due to global change and ozone depletion. We investigated the effects of UV-B (280–320 nm) and UV-A (320–400 nm) on the biogeochemically-critical filamentous marine N<sub>2</sub>-fixing cyanobacterium <i>Trichodesmium</i> (strain IMS101) using a solar simulator as well as under natural solar radiation. Short exposure to UV-B, UV-A, or integrated total UVR significantly reduced the effective quantum yield of photosystem II (PSII) and photosynthetic carbon and N<sub>2</sub> fixation rates. Cells acclimated to low light were more sensitive to UV exposure compared to high-light grown ones, which had more UV absorbing compounds, most likely mycosporine-like amino acids (MAAs). After acclimation under natural sunlight, the specific growth rate was lower (by up to 44 %), MAAs content was higher, and average trichome length was shorter (by up to 22 %) in the full spectrum of solar radiation with UVR, than under a photosynthetically active radiation (PAR) alone treatment (400–700 nm). These results suggest that prior shipboard experiments in UV-opaque containers may have substantially overestimated in-situ nitrogen fixation rates by <i>Trichodesmium</i>, and that natural and anthropogenic elevation of UV radiation intensity could significantly inhibit this vital source of new nitrogen to the current and future oligotrophic oceans.