Fuller's earth is any nonplastic clay or claylike material that can be used to decolorize, filter, and purify animal, mineral, and vegetable oils and greases. It usually has a high magnesium oxide content. In the United States, two varieties of fuller's earth are mined, mainly in the southeastern states. These comprise the minerals montmorillonite or palygorskite (attapulgite) or a mixture of the two; some of the other minerals that may be present in fuller's earth deposits are calcite, dolomite, and quartz.

The name reflects the first use of the material. In past centuries, fullers (whom we might now call dry cleaners) kneaded powdered fuller's earth into woolen garments to absorb lanolin and human body oils. When the fuller's earth was shaken out and the garment was "fulled" (fluffed), it was considered to be clean; this process is termed "fulling".

Important uses are in absorbents and filters. Because of this, fuller's earth is sometimes found in cat litter.