Previous studies have shown that reduced levels of solar UV radiation (280–400 nm) can enhance photosynthetic carbon fixation of marine phytoplankton, but the mechanisms are not known. The supply of CO₂ for photosynthesis is facilitated by extracellular (periplasmic) carbonic anhydrase (CAe) in most marine phytoplankton species. The present study showed that the CAe activity of Skeletonema costatum (Greville) Cleve was stimulated when treated with UV-A (320–395 nm) or UV-A + UV-B (295–320 nm) in addition to visible radiation. The presence of UV-A and UV-B enhanced the activity by 28% and 24%, respectively, at a low irradiance (PAR 161, UV-A 28, UV-B 0.9 W m^-2) and by 21% and 19%, respectively, at a high irradiance (PAR 328, UV-A 58, UV-B 1.9 W m^-2) level after exposure for 1 h. Ultraviolet radiation stimulated CAe activity contributed up to 6% of the photosynthetic carbon fixation as a result of the enhanced supply of CO₂, as revealed using the CAe inhibitor (acetazolamide). As a result, there was less inhibition of photosynthetic carbon fixation compared with the apparent quantum yield of PSII. The UV radiation stimulated CAe activity coincided with the enhanced redox activity at the plasma membrane in the presence of UV-A and/or UV-B. The present study showed that UV radiation can enhance CAe activity, which plays an important role in counteracting UV inhibition of photosynthesis.