The spatial and temporal distributions of Hg in different fractions of sediment samples from the Valdeazogues River (Almadén) were investigated. Almadén is widely known as the world's largest Hg mining district. A total of nine sites during six time periods were monitored using a BCR (Community Bureau of Reference) sequential extraction procedure. The results showed that Hg is mainly present as residual cinnabar but there is another important organic fraction in which Hg is primarily incorporated. Higher proportions of Hg in the residual fraction were found in wet seasons, while the opposite is true in the organic fraction. The exchangeable/carbonate and Fe-Mn oxide phases are the minor Hg fractions, and they were found less susceptible to seasonal changes. The potential risk for the aquatic medium was evaluated based on the proportions of non-residual (exchangeable/carbonate + Fe-Mn oxides + organic) fractions. The high proportions of Hg potentially (bio)available, together with the Hg total concentrations, indicated an important risk, especially in areas close to the mines. Although the risk is important in all seasons, it is particularly noticeable during dry periods due to the increase in the organic fraction, which may favor potential transference from the sediments to the environment due to the physic-chemical river conditions.