疏水效应（英语：Hydrophobic effect）又称疏水性效应，属于非极性分子的一种性质，会使这些分子在水溶液中具有自我聚集（self-associate）的特性。例如油在水中会呈现与水分离的现象。

此外这种现象对于蛋白质的型态、不同蛋白质之间的交互作用，以及核酸结构等生化分子也有所影响。例如某些拥有疏水核心（hydrophobic core）的蛋白质，会以疏水轴为中心形成卷曲螺旋（coiled-coil）结构。在水中蛋白质折叠时总是倾向于将疏水基团埋藏在分子内部，将亲水基团暴露在外部的现象。蛋白质溶液系统熵值的增加是疏水作用的主要动力，因此，疏水作用又被称作熵效应。

The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in aqueous solution and exclude water molecules. This occurs because interactions between the hydrophobic molecules enable the displaced water molecules to make hydrogen bonds more freely with each other and increase the number of hydrogen bonds they are involved with, thereby decreasing the overall free energy. The word hydrophobic literally means "water-fearing," and it describes the segregation and apparent repulsion between water and nonpolar substances.