次磷酸（化学式：H₃PO₂）是一种磷的含氧酸，也是一种很强的还原剂。无色、低熔点的晶体，易过冷成粘稠液体，可溶于水、二恶烷和乙醇。虽然次磷酸可以表示为H₃PO₂，但更准确的表达式为HOP(O)H₂，突出了其作为一元酸的特点。

HOP(O)H₂（次磷酸）中会含有少量的它的互变异构体HP(OH)₂，并保持一定的平衡。这种次要的互变异构体的IUPAC名称为“hypophosphorous acid”，有机衍生物称“亚膦酸”。

次磷酸被烃基取代后，形成的衍生物HO(O)P(R)R'称为次膦酸。

Hypophosphorous acid is a phosphorus oxoacid and a powerful reducing agent with molecular formula H₃PO₂. Inorganic chemists refer to the free acid by this name (also as "HPA"), or the acceptable name of phosphinic acid. It is a colorless low-melting compound, which is soluble in water, dioxane, and alcohols. The formula for hypophosphorous acid is generally written H₃PO₂, but a more descriptive presentation is HOP(O)H₂ which highlights its monoprotic character. Salts derived from this acid are called phosphinates (hypophosphites).