The Cassiterides, meaning Tin Islands (from the Greek word for tin: Κασσίτερος/Kassiteros), are an ancient geographical name of islands that were regarded as situated somewhere near the west coasts of Europe. The traditional assumption, ignoring Strabo, is that Cassiterides refer to Great Britain, based on the significant tin deposits in Cornwall..

Herodotus (430 BC) had only vaguely heard of the Cassiterides, "from which we are said to have our tin," but did not discount the islands as legendary. Later writers — Posidonius, Diodorus Siculus,Strabo and others — call them smallish islands off ("some way off," Strabo says) the northwest coast of the Iberian Peninsula, which contained tin mines or, according to Strabo, tin and lead mines. A passage in Diodorus derives the name rather from their nearness to the tin districts of Northwest Iberia. Ptolemy and Dionysios Periegetes mentioned them — the former as ten small islands in Northwest Iberia far off the coast and arranged symbolically as a ring, and the latter in connection with the mythical Hesperides.