连二磷酸（英语：Hypophosphoric acid），化学式H₄P₂O₆，是一种无机酸，其中磷的氧化数为+4，故具有还原性。其固体多以二水合物（H₄P₂O₆·2H₂O）形式存在。在连二磷酸分子中，两个磷原子是等价的，它们通过P-P键直接相连。它在室温下会转化为其互变异构体连二磷（Ⅲ,Ⅴ）酸（isohypophosphoric acid），这个分子的结构是一个磷原子与氢原子直接相连，氧化数为+3，而两个磷原子之间形成P-O-P键，因此另一个磷原子的氧化数为+5。连二磷（Ⅳ）酸和连二磷（Ⅲ,Ⅴ）酸在室温下都会歧化。

Hypophosphoric acid is a mineral acid with the formula H₄P₂O₆, with phosphorus in a formal oxidation state of +4. In the solid state it is present as the dihydrate, H₄P₂O₆·2H₂O. In hypophosphoric acid the P atoms are identical and joined directly with a P-P bond. There is an isomeric form isohypophosphoric acid which has a different structure with non-identical P atoms, one of which has a directly bonded H atom and an oxidation state of +3, which is joined by a P-O-P bridge to the second P atom which has an oxidation state of +5.