沉积岩，在有水循环的星球上又称为水成岩，是三种组成地球岩石圈的主要岩石之一（另外两种是岩浆岩和变质岩）。

沉积岩是在地表不太深的地方，将其他岩石的风化产物和一些火山喷发物，经过水流或冰川的搬运、沉积、成岩作用形成的岩石。在地球地表，有70%的岩石是沉积岩，但如果从地球表面到16公里深的整个岩石圈算，沉积岩只占5%，因此沉积岩是构成地壳表层的主要岩石。沉积岩主要包括有石灰岩、砂岩、页岩等。沉积岩中所含有的矿产极为丰富，占全部世界矿产蕴藏量的80%。

沉积岩特征是有层理，某些含有动植物化石，所以可以断定其地质年代。相较于火成岩及变质岩，沉积岩中的化石所受破坏较少，也较易完整保存，因此对考古学来说是十分重要的研究目标。

Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of that material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle in place. The particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, the sediment was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Sedimentation may also occur as minerals precipitate from water solution or shells of aquatic creatures settle out of suspension.