Ranunculus pusillus is a species of flowering plant in the buttercup genus known by the common name low spearwort. It is native to much of the eastern United States from New York to Florida and west to Texas; it is also known in California. It grows in wet habitat, where it is semi-aquatic growing partially submerged or terrestrially on muddy substrates. It is a perennial herb producing a slender decumbent to erect stem up to half a meter in length. It is generally hairless in texture. Leaves have blades which are lance-shaped to oval and borne on short petioles. The flower has one to three tiny yellow petals no more than 2 millimeters long around a central receptacle with many stamens and pistils. The fruit is an achene borne in a spherical cluster of 18 or more.