<p>Phosphite is a reduced inorganic phosphorus species increasingly reported in aquatic environments but often overlooked in conventional dissolved reactive phosphorus monitoring. Owing to its high solubility and redox lability, Phi can function as a transient, speciation-sensitive P pool and a substrate for microbial metabolism, thereby linking phosphorus availability to redox dynamics and potentially influencing eutrophication-relevant nutrient fluxes. Here, we synthesize current evidence on the occurrence, environmental partitioning, and transformation of Phi across various aquatic environments, and waters impacted by industrial wastewater and agricultural inputs. We first summarize recent methodological advances facilitating robust quantification of Phi at environmentally relevant concentrations. We then synthesize current knowledge of Phi biogeochemical cycling across freshwater systems, sediment and soil matrices, and marine environments, collating reported distribution patterns and disentangling the key environmental drivers governing Phi persistence and turnover dynamics. Furthermore, we delineate the core chemical and microbial transformation pathways of Phi in aquatic environments, encompassing anabolic phosphite oxidation, dissimilatory phosphite oxidation, and abiotic/engineered oxidation processes (e.g., UV/H₂O₂, Fenton-like reactions, PMS-based oxidation, and electrochemical methods). Additionally, we address the ecological and biogeochemical implications of Phi cycling for eutrophication mitigation, microbial community assembly, and aquatic ecosystem stability. Finally, we emphasized and elaborate on the priorities of future research endeavors. This work provides an in-depth and holistic understanding of the intricate phosphorus cycling processes, laying a robust scientific foundation for the design and implementation of efficient and sustainable phosphorus management strategies tailored to diverse aquatic environments.</p>