Thomas won 21 caps for his country during the 1960s and also captained Neath, who he went on to manage during the 1980s and 1990s.
A statement on the Neath RFC website read: "It is with regret that we announce that Neath legend Brian Thomas passed away in the early hours of this morning after a long illness. We would like to express our deepest sympathy to his family and friends."
Thomas, who could also play prop, won his first cap against England in 1963 and was part of the Wales sides that won the Five Nations in 1964, 1965 and 1966.
His final appearance in the red jersey came against New Zealand in Auckland in 1969.
He also won three rugby union Blues while at Cambridge University.
After his playing days were over he was credited as being the key figure behind Neath's dominance of Welsh club rugby during the late '80s and early '90s, having taken on the role of team manager in 1981.
Tributes have been paid to Thomas on Twitter with Neath MP Peter Hain writing: "Very sad death of great Neath rugby giant Brian Thomas after long illness. Sympathies to Brian's family."
Former Ospreys coach Sean Holley said: "Really sad to hear about the passing of Neath legend Brian Thomas. He was an inspiration to many. Thoughts go out to his family."