John's passing - which comes less than a month after the death of fellow Wales and Lions hero JPR Williams at 74 - was confirmed by his family in a short statement on Sunday.
"Barry John died peacefully today at the University Hospital of Wales surrounded by his loving wife and four children," the statement read.
"He was a loving Dadcu to his 11 grandchildren and much-loved brother."
John - nicknamed 'The King' and often compared to football's George Best in terms of his genius - was one of the great players in rugby union history, famed for his skills as the first true superstar during the sport's amateur era before retiring in shock fashion aged just 27.
He inspired the Lions to their famous 1971 Test series win in New Zealand, still the only Lions team ever to achieve that feat.
John appeared five times for the Lions, also playing on the 1968 tour to South Africa, and also earned 25 international caps for Wales between 1966-72, winning three Five Nations titles, a Grand Slam and two Triple Crowns.
He was part of one of the all-time great half-back partnerships along with legendary Welsh scrum-half Sir Gareth Edwards, with whom he played with all of the Lions, Wales and Cardiff.
John was also a key figure for the Barbarians and began his rugby career at Cefneithin RFC as a youngster before moving on to Llanelli and then joining Cardiff in 1967.
He is survived by his wife Janet and children Kathryn, Lucy, Anna and David, plus his 11 grandchildren.
Former club Cardiff said: "We are absolutely devastated to learn of the passing of Barry John. One of the greatest players to pull on the Blue and Black and what a partnership he shared with Sir Gareth.
"An absolute icon of who will be dearly missed. Long live the King."
We are absolutely devastated to learn of the passing of Barry John. One of the greatest players to pull on the Blue and Black and what a partnership he shared with Sir Gareth.
An absolute icon of @Cardiff_RFC , @WelshRugbyUnion and @lionsofficial who will be dearly missed.… pic.twitter.com/6NTtzjnlpT
— Cardiff Rugby (@Cardiff_Rugby) February 4, 2024
“To be crowned ‘The King’ in New Zealand when every back row forward in both the North and South Islands is trying to take your head off is quite some accolade,” said Terry Cobner, president of the Welsh Rugby Union (WRU).
“For me, he has got to be right up there among the greatest outside halves who have ever played the game – probably the greatest.
“He was a glider, rather than a sidestepper, who had a subtle change of pace and direction. Coming on top of the recent deaths of Brian Price and JPR Williams, this is another huge blow for Welsh rugby.
“After what he did for Wales and the Lions in 1971, those of us who followed him into both teams always felt we had huge shoes to fill. He was and will remain a legend of our game.”
Truly one of the greatest.
We are hugely saddened that the great Barry John has passed away at the age of 79.
Barry inspired so many and will forever be remembered for how much he gave to the sport.
All our thoughts are with his family and friends.
Rest in Peace. pic.twitter.com/wyI9ZL4FVu
— British & Irish Lions (@lionsofficial) February 4, 2024
In a statement, the Lions said: "Truly one of the greatest. We are hugely saddened that the great Barry John has passed away at the age of 79.
"Barry inspired so many and will forever be remembered for how much he gave to the sport. All our thoughts are with his family and friends. Rest in Peace."