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.^[1] He used the newspaper to campaign for the disestablishment of the Church in Wales. Rees also penned the hymn text of Dyma gariad fel y moroedd (Here is love, vast as the ocean), which was first published in 1847 but strongly associated with the 1904-1905 Welsh revival.^[2] His Helyntion Bywyd Hen Deiliwr (Predicaments of an Old Tailor) (1877) was a pioneering attempt to fashion a Welsh-language novel.^[3]