Neptunism is a superseded scientific theory of geology proposed by Abraham Gottlob Werner (1749–1817) in the late 18th century, who proposed that rocks formed from the crystallisation of minerals in the early Earth's oceans.

The theory took its name from Neptune, the ancient Roman god of the sea. There was considerable debate between its proponents (neptunists) and those favouring a rival theory known as plutonism which gave a significant role to volcanic origins, and which in modified form replaced neptunism in the early 19th century as the principle of uniformitarianism was shown to fit better with the geological facts as they became better known.

Modern geology acknowledges many different forms of rock formation, and explains the formation of sedimentary rock through processes very similar to those described by neptunism.