Percy Bysshe Shelley 4 August 1792 – 8 July 1822) was one of the major English Romantic poets, and is regarded by some as among the finest lyric, as well as epic, poets in the English language. A radical in his poetry as well as in his political and social views, Shelley did not see fame during his lifetime, but recognition for his poetry grew steadily following his death. Shelley was a key member of a close circle of visionary poets and writers that included Lord Byron, Leigh Hunt, Thomas Love Peacock, and his own second wife, Mary Shelley, the author of Frankenstein.

Shelley is perhaps best known for such classic poems as Ozymandias, Ode to the West Wind, To a Skylark, Music, When Soft Voices Die, The Cloud and The Masque of Anarchy. His other major works include a groundbreaking verse drama The Cenci (1819) and long, visionary poems such as Queen Mab (later reworked as The Daemon of the World), Alastor, The Revolt of Islam, Adonaïs, Prometheus Unbound (1820)—widely considered to be his masterpiece—Hellas: A Lyrical Drama (1821), and his final, unfinished work, The Triumph of Life (1822).

Shelley's close circle of friends included some of the most important progressive thinkers of the day, including his father-in-law, the philosopher William Godwin and Leigh Hunt. Though Shelley's poetry and prose output remained steady throughout his life, most publishers and journals declined to publish his work for fear of being arrested for either blasphemy or sedition. Shelley's poetry sometimes had only an underground readership during his day, but his poetic achievements are widely recognized today, and his advanced political and social thought impacted the Chartist and other movements in England, and reach down to the present day. Shelley's theories of economics and morality, for example, had a profound influence on Karl Marx; his early—perhaps first—writings on nonviolent resistance influenced both Leo Tolstoy and Mahatma Gandhi.

Shelley became a lodestone to the subsequent three or four generations of poets, including important Victorian and Pre-Raphaelite poets such as Robert Browning and Dante Gabriel Rossetti. He was admired by Oscar Wilde, Thomas Hardy, George Bernard Shaw, Bertrand Russell, W. B. Yeats, Upton Sinclair and Isadora Duncan. Henry David Thoreau's civil disobedience was apparently influenced by Shelley's non-violence in protest and political action. Shelley's popularity and influence has continued to grow in contemporary poetry circles.

On 8 July 1822, less than a month before his 30th birthday, Shelley drowned in a sudden storm on the Gulf of Spezia while returning from Leghorn (Livorno) to Lerici in his sailing boat, the Don Juan. He was returning from having set up The Liberal with the newly arrived Leigh Hunt. The name Don Juan, a compliment to Byron, was chosen by Edward John Trelawny, a member of the Shelley-Byron Pisan circle. However, according to Mary Shelley's testimony, Shelley changed it to Ariel, which annoyed Byron, who forced the painting of the words "Don Juan" on the mainsail. The vessel, an open boat, was custom-built in Genoa for Shelley. It did not capsize but sank; Mary Shelley declared in her "Note on Poems of 1822" (1839) that the design had a defect and that the boat was never seaworthy. In fact the Don Juan was seaworthy; the sinking was due to a severe storm and poor seamanship of the three men on board.

Some believed his death was not accidental, that Shelley was depressed and wanted to die; others suggest he simply did not know how to navigate. More fantastical theories, including the possibility of pirates mistaking the boat for Byron's, also circulated. There is a small amount of material, though scattered and contradictory, suggesting that Shelley may have been murdered for political reasons: previously, at Plas Tan-Yr-Allt, the Regency house he rented at Tremadog, near Porthmadog, north-west Wales, from 1812 to 1813, he had allegedly been surprised and attacked during the night by a man who may have been, according to some later writers, an intelligence agent. Shelley, who was in financial difficulty, left forthwith leaving rent unpaid and without contributing to the fund to support the house owner, William Madocks; this may provide another, more plausible explanation for this story.