Solar radiation in general and UV radiation in particular have been recognized to stimulate plant litter decomposition through photochemical mineralization of organic molecules such as lignin and through facilitation of microbial decomposition in dryland ecosystems. However, little is known about how photodegradation may influence decomposition in other ecosystems not subject to moisture limitations and under what conditions photodegradation may be favored. Decomposition in humid tropical ecosystems is a complex process, and it can be influenced by a number of environmental factors that are distinct from arid and semi-arid ecosystems. To assess the mechanisms underlying photodegradation by ultraviolet B (UV-B) radiation in a humid tropical ecosystem, we designed a 300-day field experiment in a tropical site in Brazil with high levels of annual precipitation, compared to arid ecosystems, and exposed litter to three levels of radiation (full sun, UV-B removed, and shade) combined with a biocide treatment. Results show that after nearly one year of exposure, the microbial biomass was not affected by UV-B incidence, and this effect has not yet been fully understood for tropical ecosystems. Modeled using an exponential deceleration equation, the removal of UV-B radiation decelerated the plant litter decomposition rate for the control conditions by 21% compared to litter exposed to full sun. Interestingly, shaded litter exhibited similar mass loss compared to litter exposed to full sun. Furthermore, differences in the decay constant among radiation treatments due to the UV-B effect were independent of lignin loss. Overall, our study suggests that UV-B radiation contributed to plant litter decomposition through carbon losses but had no discernible effect on nitrogen, lignin, or cellulose loss specifically. Importantly, our results demonstrate that photodegradation occurs under humid tropical conditions, and further studies are necessary to examine the mechanisms of carbon loss.