"Barry John died peacefully today at the University Hospital of Wales surrounded by his loving wife and four children," a family statement said.
John made his Wales debut in 1966, making 25 international appearances and scoring 90 points. He also played five Tests for the Lions, and was a vital part of their 2-1 series win in New Zealand in 1971.
"Truly one of the greatest," the British & Irish Lions said in a post on X (formerly known as Twitter). "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."
Wales won the Grand Slam in 1971, their first since 1952, with John, nicknamed "The King," scoring a try and a penalty in the 9-5 win over France which sealed the Championship success.
He retired in 1972 at 27, after scoring 35 points in that year's Five Nations, with Wales denied another chance of a Grand Slam as both Wales and Scotland refused to travel to face Ireland after an escalation of violence in Northern Ireland.
John is the second Welsh rugby star of the 1970s to die this year, following the death of JPR Williams in January.