S4C (Welsh pronunciation: [ˌɛs ˌpɛdwar ˈɛk], Sianel Pedwar Cymru, meaning Channel Four Wales) is a Welsh language free-to-air public broadcast television channel. Launched on 1 November 1982, it was the first television channel to be aimed specifically at a Welsh-speaking audience. S4C's headquarters are based in Carmarthen, at the University of Wales Trinity Saint David's creative and digital centre, Yr Egin. It also has regional offices in Caernarfon and Cardiff. As of 2024, S4C had an average of 118 employees. S4C is the fourth-oldest terrestrial television channel in Wales after BBC One, ITV and BBC Two.

As with Channel 4 (which launched the next day in the rest of the UK), S4C commissions all of its programmes from independent producers. BBC Cymru Wales produces programmes for S4C as part of its public service remit, including the news service Newyddion. From its launch until 2010, S4C also carried English-language programming acquired from Channel 4, which could not be received over-the-air in most of Wales; these programmes aired in non-peak hours and did not always air in pattern with Channel 4's scheduling.

S4C has been described by Elin Haf Gruffydd Jones as a "trailblazer" in European broadcasting for minority languages, going on air less than two months before Euskal Telebista (31 December 1982), TV3 Catalonia (test/trial broadcast on 11 September 1983, regular programming began in 1984) and Televisión de Galicia (24 June 1985), the first Spanish regional television stations to go on air, symbolically, in non-Castillan Spanish areas, and far ahead of other Celtic-language services, Ireland's TG4 (formerly TnaG) (31 October 1996), the ill-fated Sottish Gaelic TeleG (1999–2011) and BBC Alba (19 September 2008).