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.