PIANOSA (anc. Planasia), an island of Italy, belonging to the province of Leghorn, and forming part of the commune of Marciana (Elba), from which it is 712 m. S.W. Pop. (1881), 774. As its name indicates, it is quite flat, and the highest point is only 95 ft. above sea-level. Its area is 6 sq. m. Augustus banished to it his grandson, Agrippa Postumus, and some ruins of baths near the harbour still bear his name. It changed hands more than once in the wars between Pisa and Genoa in the 12th and 13th centuries, from 1390 it belonged to the prince of Piombino, but was depopulated in 1553 by the Turkish fleet, and only resettled at the beginning of the 19th century. In 1857 a penal colony was established here.