S4C (Welsh pronunciation: [ɛs ˈpɛdwar ɛk], from the Welsh Sianel Pedwar Cymru, meaning "Channel Four Wales") is a Welsh-language public-service television channel based in Cardiff and broadcast throughout the United Kingdom.

The first television channel to be aimed specifically at a Welsh-speaking audience, S4C is (after BBC One, ITV, and BBC Two) the fourth-oldest television channel in the United Kingdom (Channel 4 was launched in the rest of the country one day later.)

Its chief executive since April 2012 is Ian Jones. Owen Evans, currently deputy permanent secretary to the Welsh Government, will succeed Ian Jones in October 2017.

When first established, the channel—initially broadcast on analogue television—was bilingual (Welsh and English) outside peak hours, with English-language content consisting of the simultaneous or deferred transmission of programmes from Channel 4 (analogue reception of which was unavailable in most of Wales). When digital television arrived several years later, S4C added a second, 100% Welsh-language service, called S4C Digidol ("digital"). With the completion of the digital switchover in Wales on 31 March 2010—which made English-language Channel 4 available across Wales—S4C's bilingual analogue channel closed, and what had been S4C Digidol became the default S4C channel, available on Freeview, satellite and cable, and broadcasting entirely in Welsh. S4C does not commission programming in English, but when English is used on the channel it is left untranslated.

A high-definition service called S4C Clirlun ("clear picture"), simulcasting S4C's main channel, began transmission on 30 April 2010 on Freeview channel 53 in Wales. However, it was announced on 11 July 2012 that, as part of cost-saving measures designed to deal with the impact of cuts to S4C's public funding, the Clirlun channel would close before the end of the year. Channel 4 HD began broadcasting in Wales from 2 December 2012 in its place.