Uranium is a naturally occurring radioactive element that has emerged as a significant environmental contaminant, particularly due to its increasing presence in water sources used for human consumption. It has been widely detected and reported in groundwater, surface water, and even public water supplies across various regions globally. The solubility of uranium in water and its mobility and bioavailability are governed primarily by the pH of the aqueous medium. Changes in water pH, influenced by both natural processes and anthropogenic activities, can significantly affect the chemical behavior of uranium, facilitating its release from rocks, soils, and other environmental matrices. This chapter focuses on the critical role of pH in understanding uranium solubility, with particular emphasis on uranium in its hexavalent oxidation state (U(VI)), which is highly soluble and environmentally mobile. This highlights various uranium-bearing minerals, their geochemical interactions, and the pH-dependent formation of uranium complexes, especially carbonate and hydroxide species. On the basis of global data on uranium concentrations and a review of the mechanisms that control its solubility and speciation, this chapter highlights how pH acts as a primary regulatory factor in uranium transport, distribution, and health risk. Furthermore, understanding these pH-dependent mechanisms is essential for developing effective environmental monitoring and remediation strategies to mitigate uranium contamination.