成岩作用是指沉积物在埋藏后，固结为坚硬岩石，受到变质、风化作用前的各种物理、化学、生物的变化。其中沉积物变为沉积岩的变化，是狭义的成岩作用。

沉积后，沉积物连续埋藏，被溶解物中的矿物胶结。沉积物的颗粒，岩石碎片、化石可在成岩时被其它矿物交代。在成岩时孔隙度常会减小，除矿物溶解、白云石化作用。研究岩石的成岩作用常用于了解它们的构造史，及通过它们的流体的性质和类型。从商业角度，这些研究可帮助评估发现各种经济上可行的矿藏的可能性以及石油资源。 成岩作用过程同样在骨头的分解中有重要作用。

Diagenesis /ˌdaɪəˈdʒɛnəsɪs/ is the change of sediments or existing sedimentary rocks into a different sedimentary rock during and after rock formation (lithification), at temperatures and pressures less than that required for the formation of metamorphic rocks. It does not include changes from weathering. It is any chemical, physical, or biological change undergone by a sediment after its initial deposition, after its lithification. This process excludes surface alteration (weathering) and metamorphism. These changes happen at relatively low temperatures and pressures and result in changes to the rock's original mineralogy and texture. There is no sharp boundary between diagenesis and metamorphism, but the latter occurs at higher temperatures and pressures than the former. Hydrothermal solutions, meteoric groundwater, porosity, permeability, solubility, and time are all influential factors.