William Rees (8 November 1802 – 8 November 1883), usually known in Wales by his bardic name of Gwilym Hiraethog, was a Welsh poet and author, one of the major figures of Welsh literature during the 19th century.
Gwilym Hiraethog took his pseudonym from his birthplace, a farm on the Hiraethog mountain in Denbighshire.
Largely self-educated, he was a polymath, who took an interest in astronomy and political science as well as being a Nonconformist minister and a leading literary figure.
In 1843 he founded the Welsh language journal Yr Amserau ("The Times") in Liverpool.
He used the newspaper to campaign for the disestablishment of the Church in Wales.
His Helyntion Bywyd Hen Deiliwr (Predicaments of an Old Tailor) (1877) was a pioneering attempt to fashion a Welsh-language novel.