Sir John Kyffin Williams, OBE, RA (9 May 1918 – 1 September 2006) was a Welsh landscape painter who lived at Pwllfanogl, Llanfairpwll, on the Island of Anglesey. Williams is widely regarded as the defining artist of Wales during the 20th century.

Williams was born in Llangefni, Anglesey, one of two sons into an old landed Anglesey family. His father was a bank manager. Williams wrote that his mother was an emotionally repressed woman who had a virulent dislike of the Welsh and the Welsh language. Kyffin Williams was educated at Moreton Hall School, Trearddur House School in Anglesey, then at Shrewsbury School where he contracted polio encephalitis which led him to develop epilepsy, a misfortune he later described as "my greatest fortune". He joined the 6th Battalion Royal Welch Fusiliers as a lieutenant in 1937. After he failed a British Army medical examination in 1941 (because of epilepsy), the examining doctor suggested he pursue his interest in art.

Williams enrolled at London's Slade School of Fine Art in 1941 (relocated to Oxford during the war), gaining prizes for portraiture at the end of both his second and third years. He then achieved his ambition to teach art by accepting a position at Highgate School, London, where he was senior art master from 1944 until 1973. His pupils included the historian Sir Martin Gilbert, Royal Academicians Anthony Green and Patrick Procktor and composers John Tavener and John Rutter. He painted a portrait of the school's cricket coach, former test player Albert Knight. Wales never left his consciousness or imagination, as he would return home in holidays, take his study sketches back to London and complete his canvasses. Williams' particular technique was applying thick oil paint with a palette knife. Williams was predominately a landscape painter but he also was an accomplished portraitist and something of a cartoonist.

In 1968 he won a Winston Churchill Fellowship to study and paint in Y Wladfa, the Welsh settlement in Patagonia, South America. This added a significant body of work to his oeuvre (nearly 50 Patagonia paintings) and the light in Patagonia radically changed his palette.

On retiring from Highgate school Williams returned to Anglesey and spent the next 30 years painting, and promoting Welsh schools of Art and Welsh art in general.

Never having married, Williams died, without heirs, on 1 September 2006, aged 88, at St Tysilio Nursing Home on Anglesey. He had been at the home on the outskirts of Llanfairpwll for just over a week after spending two weeks as a patient at Ysbyty Gwynedd, Bangor. He had been suffering from prostate cancer and from lung cancer, believed to be a result of working with lead-based paints. Williams was buried at St Mary's Church, Llanfair-yng-Nghornwy. Paying tribute to Williams after his death, bass-baritone singer and Williams collector Bryn Terfel said, "I'm deeply saddened by the passing of Wales's foremost ambassador in the visual arts. Long may his memory live on in the legacy of his numerous, wonderful paintings."

In 1949 Williams had his first one-man-show at a private gallery in London. His first Patagonia exhibition took place in 1971. Later in 2006, the Welsh singer and Manic Street Preachers frontman James Dean Bradfield included a track called "Which Way to Kyffin", dedicated to Williams, on his album The Great Western.

His works typically drew inspiration from the Welsh landscape and farmlands. His works may be seen in a permanent exhibition in the Oriel Kyffin Williams Gallery which opened in 2008 at Oriel Ynys Môn in Llangefni, Anglesey, as well as at many other galleries elsewhere in Britain. He was president of the Royal Cambrian Academy and was appointed a member of the Royal Academy in 1974.