The Cassiterides (Ancient Greek: Κασσιτερίδες, meaning "tin place", from κασσίτερος, kassíteros "tin") are an ancient geographical name used to refer to a group of islands whose precise location is unknown, but which was believed to be situated somewhere near the west coast of Europe.

Κασσιτερίδας simply means "places of tin", but Herodotus (5.115) describes them as "νήσους οἶδα Κασσιτερίδας" or "tin islands". Modern theories hold the islands to be the British Isles.

Herodotus, whilst a subject of the Achaemenid empire in 5th century BC Halicarnassus, wrote in Ancient Greek about how his civilization received its tin and amber from western Europe, with whose geography he was unfamiliar (Hist. 3.115):

αὗται μέν νυν ἔν τε τῇ Ἀσίῃ ἐσχατιαί εἰσι καὶ ἐν τῇ Λιβύῃ. περὶ δὲ τῶν ἐν τῇ Εὐρώπῃ τῶν πρὸς ἑσπέρην ἐσχατιέων ἔχω μὲν οὐκ ἀτρεκέως λέγειν· οὔτε γὰρ ἔγωγε ἐνδέκομαι Ἠριδανὸν καλέεσθαι πρὸς βαρβάρων ποταμὸν ἐκδιδόντα ἐς θάλασσαν τὴν πρὸς βορέην ἄνεμον, ἀπʼ ὅτευ τὸ ἤλεκτρον φοιτᾶν λόγος ἐστί, οὔτε νήσους οἶδα Κασσιτερίδας ἐούσας, ἐκ τῶν ὁ κασσίτερος ἡμῖν φοιτᾷ.

English translation:

These then are the most distant lands in Asia and Libya. But concerning those in Europe that are the farthest away towards evening, I cannot speak with assurance; for I do not believe that there is a river called by foreigners Eridanus issuing into the northern sea, where our amber is said to come from, nor do I have any knowledge of Tin Islands, where our tin is brought from.

Strabo's passage on the Cassiterides is quite detailed, placing them in an Iberian context, separate from Britain (Geog. 3.5.11):

The Cassiterides are ten in number, and lie near each other in the ocean towards the north from the haven of the Artabri. One of them is desert, but the others are inhabited by men in black cloaks, clad in tunics reaching to the feet, girt about the breast, and walking with staves, thus resembling the Furies we see in tragic representations. They subsist by their cattle, leading for the most part a wandering life. Of the metals they have tin and lead; which with skins they barter with the merchants for earthenware, salt, and brazen vessels. Formerly the Phoenicians alone carried on this traffic from Gades, concealing the passage from every one; and when the Romans followed a certain ship-master, that they also might find the market, the shipmaster of jealousy purposely ran his vessel upon a shoal, leading on those who followed him into the same destructive disaster; he himself escaped by means of a fragment of the ship, and received from the state the value of the cargo he had lost. The Romans nevertheless by frequent efforts discovered the passage, and as soon as Publius Crassus, passing over to them, perceived that the metals were dug out at a little depth, and that the men were peaceably disposed, he declared it to those who already wished to traffic in this sea for profit, although the passage was longer than that to Britain. Thus far concerning Iberia and the adjacent islands.