The Twelfth Letter of Plato, also known as Epistle XII or Letter XII, is an epistle that tradition has ascribed to Plato, though it is almost certainly a literary forgery. Of all the Epistles, it is the only one that is followed by an explicit denial of its authenticity in the manuscripts. In the Stephanus pagination, it spans 359c–e of Vol. III.
Like the Ninth Letter, the Twelfth Letter is purportedly addressed to Archytas. It thanks him for sending Plato some treatises, which it then goes on to praise effusively, declaring its author worthy of his ancestors and including in their number Myrians, colonists from Troy during the reign of Laomedon. It then promises to send to Archytas some of Plato's unfinished treatises.
Diogenes Laertius preserves this letter in his Lives and Opinions of Eminent Philosophers, as well as a letter from Archytas which presumably occasioned the Twelfth Letter; This letter points to the treatises having been those of Ocellos of Lucania, a Pythagorean. Because the writings which are attributed to Ocellos are forgeries from the First Century BCE, the Twelfth Letter is probably also a forgery, and by the same forger, intended to stamp the treatises with Plato's authority. There is no other mention of a Trojan colony in Italy from the reign of Laomedon, let alone of Lucania or the Lucani having been descended from the otherwise unknown "Myrians." R. G. Bury also notes that the Twelfth Letter, along with the Ninth, spell Archytas with an α, whereas Plato spells it in more authoritative epistles with an η (Αρχύτης).