磷酸二酯键（英语：phosphodiester bond）也称为“3',5'-磷酸二酯键”或“磷酸双酯键”，是核酸分子中的磷酸基团的磷原子与另外两个五碳糖分子的碳原子之间形成的共价键。这种形式的键结于DNA及RNA分子中负责将分别位于两个核糖上的3号碳与5号碳链接起来。

A phosphodiester bond occurs when exactly two of the hydroxyl groups in phosphoric acid react with hydroxyl groups on other molecules to form two ester bonds.

Phosphodiester bonds are central to all life on Earth, as they make up the backbone of the strands of nucleic acid. In DNA and RNA, the phosphodiester bond is the linkage between the 3' carbon atom of one sugar molecule and the 5' carbon atom of another, deoxyribose in DNA and ribose in RNA. Strong covalent bonds form between the phosphate group and two 5-carbon ring carbohydrates (pentoses) over two ester bonds.