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A protolith is the original, unmetamorphosed rock from which a given metamorphic rock is formed (proto-: first; lithos: rock; both Greek).[1]

For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock and thus there is a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.

Protoliths are non-metamorphic rocks and thus have no protoliths themselves. The non-metamorphic rocks fall into two classes: sedimentary rocks, formed from sediment, and igneous rocks, formed from magma. The source of the sediment of a sedimentary rock is termed its provenance.