Ossian (/ˈɒʃən, ˈɒsiən/; Irish Gaelic/Scottish Gaelic: Oisean) is the narrator and purported author of a cycle of epic poems published by the Scottish poet James Macpherson, originally as Fingal (1761) and Temora (1763), and later combined under the title The Poems of Ossian. Macpherson claimed to have collected word-of-mouth material in Scottish Gaelic, said to be from ancient sources, and that the work was his translation of that material. Ossian is based on Oisín, son of Fionn mac Cumhaill (anglicised to Finn McCool), a legendary bard in Irish mythology. Contemporary critics were divided in their view of the work's authenticity, but the current consensus is that Macpherson largely composed the poems himself, drawing in part on traditional Gaelic poetry he had collected.

The work was internationally popular, translated into all the literary languages of Europe, and was highly influential both in the development of the Romantic movement and the Gaelic revival. Macpherson's fame was crowned by his burial among the literary giants in Westminster Abbey. W. P. Ker, in the Cambridge History of English Literature, observes that "all Macpherson's craft as a philological impostor would have been nothing without his literary skill."

In 1760, Macpherson published the English-language text Fragments of ancient poetry, collected in the Highlands of Scotland, and translated from the Gaelic or Erse language. Later that year, he claimed to have obtained further manuscripts and in 1761 he claimed to have found an epic on the subject of the hero Fingal (with Fingal or Fionnghall meaning 'fair stranger' denoting hair or eye colour ), written by Ossian. According to Macpherson's prefatory material, his publisher, claiming that there was no market for these works except in English, required that they be translated. Macpherson published these alleged translations during the next few years, culminating in a collected edition, The Works of Ossian, in 1765. The most famous of these Ossianic poems was Fingal, written in 1761 and dated 1762.

The supposed original poems are translated into poetic prose, with short and simple sentences. The mood is epic, but there is no single narrative, although the same characters reappear. The main characters are Ossian himself, relating the stories when old and blind, his father Fingal (very loosely based on the Irish hero Fionn mac Cumhaill), his dead son Oscar (also with an Irish counterpart), and Oscar's lover Malvina (like Fiona a name invented by Macpherson), who looks after Ossian in his old age. Though the stories "are of endless battles and unhappy loves", the enemies and causes of strife are given little explanation and context.

Characters are given to killing loved ones by mistake, and dying of grief, or of joy. There is very little information given on the religion, culture or society of the characters, and buildings are hardly mentioned. The landscape "is more real than the people who inhabit it. Drowned in eternal mist, illuminated by a decrepit sun or by ephemeral meteors, it is a world of greyness." Fingal is king of a region of south-west Scotland perhaps similar to the historical kingdom of Dál Riata and the poems appear to be set around the 3rd century, with the "king of the world" mentioned being the Roman Emperor; Macpherson and his supporters detected references to Caracalla (d. 217, as "Caracul") and Carausius (d. 293, as "Caros", the "king of ships").

The poems achieved international success. Napoleon and Diderot were prominent admirers, and Voltaire was known to have written parodies of them. Thomas Jefferson thought Ossian "the greatest poet that has ever existed", and planned to learn Gaelic so as to read his poems in the original. They were proclaimed as a Celtic equivalent of the Classical writers such as Homer. "The genuine remains of Ossian ... are in many respects of the same stamp as the Iliad", was Thoreau's opinion. Many writers were influenced by the works, including Walter Scott, and painters and composers chose Ossianic subjects.

The Hungarian national poet Sándor Petőfi wrote a poem entitled Homer and Ossian, comparing the two authors, of which the first verse reads:

Oh where are you Hellenes and Celts?
Already you have vanished, like
Two cities drowning
In the waters of the deep.
Only the tips of towers stand out from the water,
Two tips of towers: Homer, Ossian.