Euphorbia herbstii (syn. Chamaesyce herbstii) is a rare species of flowering plant in the family Euphorbiaceae known by the common name Herbst's sandmat. It is endemic to Oʻahu, Hawaii, where it is rapidly disappearing. Like other Hawaiian euphorbs, this plant is known locally as ʻakoko. It is a federally listed endangered species of the United States. This plant is usually a tree which can grow up to 8 meters tall. The leaf blades are long and narrow, and the branches bleed a milky sap. The inflorescence is a cluster of cyathia.