Piastre atau piaster (bahasa Inggris: pengucapan: /piˈæstər/) adalah sebuah jumlah unit mata uang. Istilah tersebut berasal dari kata Italia untuk "lempeng metal tipis". Nama tersebut diterapkan kepada keping delapan Spanyol dan Amerika Hispanik, atau peso, oleh para pedagang Venesia di Syam pada abad ke-16.
Eckfeldt, Jacob Reese; Du Bois, William Ewing; Saxton, Joseph (1842). A manual of gold and silver coins of all nations, struck within the past century. Showing their history, and legal basis, and their actual weight, fineness, and value chiefly from original and recent assays. With which are incorporated treatises on bullion and plate, counterfeit coins, specific gravity of precious metals, etc., with recent statistics of the production and coinage of gold and silver in the world, and sundry useful tables. Assay Office of the Mint. hlm. 132.