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.