Ictis (Ancient Greek: Ἴκτιν, romanized: Íktin) was a British island described as a tin trading centre in the Bibliotheca historica of the Sicilian-Greek historian Diodorus Siculus, writing in the first century BC.

While Ictis is widely accepted to have been an island somewhere off the southern coast of what is now England, scholars continue to debate its precise location. Candidates include St Michael's Mount and Looe Island off the coast of Cornwall, the Mount Batten peninsula and Burgh Island in Devon, Hengistbury Head in Dorset and the Isle of Wight further to the east.

The most detailed description of Ictis is found in the Bibliotheca Historica (5.22) of Diodorus Siculus, written in ancient Greek. This was first translated into English by George Booth (1700; reprinted 1814), but C. H. Oldfather's translation of 1939 is more often used:

But we shall give a detailed account of the customs of Britain and of the other features which are peculiar to the island when we come to the campaign which Caesar undertook against it, and at this time we shall discuss the tin which the island produces. The inhabitants of Britain who dwell about the promontory known as Belerium are especially hospitable to strangers and have adopted a civilized manner of life because of their intercourse with merchants of other peoples. They it is who work the tin, treating the bed which bears it in an ingenious manner. This bed, being like rocks contains earthy seams and in them the workers quarry the ore which they then melt down and cleanse of its impurities. Then they work the tin into pieces the size of knuckle-bones and convey it to an island which lies off Britain and is called Ictis for at the time of ebb-tide the space between this island and the mainland becomes dry and they can take the tin in large quantities over to the island on their wagons. (And a peculiar thing happens in the case of the neighbouring islands which lie between Europe and Britain, for at flood-tide the passages between them and the mainland run full and they have the appearance of islands, but at ebb-tide the sea recedes and leaves dry a large space, and at that time they look like peninsulas.) On the island of Ictis the merchants purchase the tin of the natives and carry it from there across the Strait to Galatia or Gaul; and finally, making their way on foot through Gaul for some thirty days, they bring their wares on horseback to the mouths of the river Rhone.

A more recent English translation by Lionel Scott appeared in 2022 and one by Casevitz & Jacquemin into French in 2015.

In the Greek text of Diodorus, the name appears, in the accusative case, as "Iktin", so that translators have inferred that the nominative form of the name was "Iktis", rendering this into the medieval lingua franca of Latin (which only rarely used the letter 'k') as "Ictis". However, some commentators doubt that "Ictis" is correct and prefer "Iktin".

Diodorus Siculus, who flourished between about 60 and about 30 BC, is supposed to have relied for his account of the geography of Britain on a lost work of Pytheas, a Greek geographer from Massalia who made a voyage around the coast of Britain near the end of the fourth century BC, searching for the source of amber. The record of the voyage of Pytheas was lost in antiquity but was known to some later writers, including Timaeus, Posidonius, and Pliny the Elder. Their work is contradictory, but from it deductions can be made about what was reported by Pytheas. This "represents all that was known about the tin trade in the ancient classical world".

Scott (2022) gives Diodorus 5.22 as F5 in his study of Pytheas's fragments; but only as a secondary source referenced via Timaeus (FGrH 566 F164.22). Unlike Strabo and Pliny, Diodorus never explicitly cites his sources, however there are several clues that his source was Timaeus: