The former Wales and British and Irish Lions five-eighth died peacefully in hospital, his family announced on Sunday (Monday AEDT).
John, nicknamed 'The King' by New Zealand journalists after he inspired the Lions' famous 1971 Test series victory in the den of the All Blacks, won 25 Wales caps between 1966 and 1972.
He also played in five Lions Tests on their 1968 and 1971 tours, before retiring from rugby at the age of 27.
A statement released by John's family read: "Barry John died peacefully today at the University Hospital of Wales surrounded by his loving wife and four children.
"He was a loving dad to his 11 grandchildren and much-loved brother."
John played his club rugby for Llanelli and then Cardiff, where he struck up a matchless half-back partnership with Gareth Edwards that went on to flourish for both Wales and the Lions.
John was partnered by Edwards in 23 of his Wales international appearances, plus all five Lions Tests - one against South Africa and four against New Zealand.
It's an immense blow to Welsh rugby just four weeks after another star of their golden era of the 1970s, fullback JPR Williams, died.
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