软硬酸碱理论简称HSAB（英语：Hard-Soft-Acid-Base）理论，是一种尝试解释酸碱反应及其性质的现代理论。20世纪60年代初，拉尔夫·皮尔逊介绍了HSAB的原理，作为一种尝试，用以统一有机和无机化学反应。它目前在化学研究中得到了广泛的应用，其中最重要的莫过于对配合物稳定性的判别和其反应机理的解释。软硬酸碱理论的基础是酸碱电子论，即以电子对得失作为判定酸、碱的标准（即路易斯酸碱理论）。该理论可用于定性描述，而非定量的描述，这将有助于了解化学性质和反应的主要驱动因素。尤其是在过渡金属化学，化学家们已经完成了无数次实验，以确定配体和过渡金属离子本身的硬和软方面的相对顺序。

The HSAB concept is an initialism for "hard and soft (Lewis) acids and bases". Also known as the Pearson acid base concept, HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical species. 'Hard' applies to species which are small, have high charge states (the charge criterion applies mainly to acids, to a lesser extent to bases), and are weakly polarizable. 'Soft' applies to species which are big, have low charge states and are strongly polarizable. The concept is a way of applying the notion of orbital overlap to specific chemical cases.