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 2019–20, S4C had an average of 101 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.

On digital terrestrial television, S4C has broadcast exclusively in Welsh since the platform's launch in 1998, with the existing bilingual schedule continuing on analogue television. After the completion of the digital switchover in Wales on 31 March 2010, Channel 4 became available on Freeview, and S4C ceased its carriage of English-language programmes. S4C offers translated, English-language subtitles for its Welsh programming.