Baron Merthyr, of Senghenydd in the County of Glamorgan, is a title in the Peerage of the United Kingdom. It was created in 1911 for the Welsh coal mining magnate Sir William Lewis, 1st Baronet. He had already been created a Baronet, of Nantgwyne in the County of Glamorgan, in 1896. The barony is named after the town of Merthyr Tydfil, where Lewis was born. Lord Merthyr's grandson, the third Baron, was Chairman of Committees in the House of Lords from 1957 to 1965 and a Deputy Speaker from 1957 to 1974. The latter was succeeded by his son, the fourth Baron. He disclaimed the peerage for life on 26 April 1977, three weeks after succeeding his father. He does not use his title of Baronet either.
The Hon. Robin Lewis, fourth son of the third Baron, was appointed Lord Lieutenant of Dyfed in December 2006, although this was not officially reported in the London Gazette until September 2007.
The heir apparent is the present the holder's son, David Trevor Lewis (b. 1977)