Calcium phosphite (CaHPO₃) is an inorganic salt of phosphorous acid. It is typically a white to off-white powder with low solubility in water. The compound is recognized for its role as a source of phosphite ions (HPO₃²⁻). Its primary and well-established application is in agriculture as a systemic fungicide and bactericide. It is used to control a variety of plant diseases, including those caused by oomycetes such as *Phytophthora* and *Pythium* species. The phosphite ion is believed to act both directly on the pathogen and by stimulating the plant's own defense mechanisms (systemic acquired resistance). It is also utilized as a source of phosphorus fertilizer, particularly in situations where phosphate availability is limited. Beyond its agricultural uses, calcium phosphite serves as a chemical intermediate and a reducing agent in certain inorganic synthesis processes. It is also investigated for its potential as a flame retardant additive in polymers and as a corrosion inhibitor.