Ultramafic rocks are igneous rocks with very low silica content (less than 45%) and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). Ultramafic rocks are typical of the Earth's mantle.

Rock types include intrusive dunite and peridotite and rare volcanic komatiite and picrite . Most surface exposures of ultramafic rocks occur in ophiolite complexes where deep mantle derived rocks have been obducted into continental crust along and above subduction zones.

Where ultramafic rocks are exposed on the surface, the high metal content of the rocks creates unique vegetation. Examples are the Ultramafic woodlands and Ultramafic barrens of the Appalachian mountains and piedmont, the "wet maquis" of the New Caledonia rain forests, and the Ultramafic forests of Mount Kinabalu and other peaks in Sabah, Malaysia. Vegetation is typically stunted, and is sometimes home to endemic species adapted to the metallic soils.