Thomas made 357 appearances for the Swans between 1965 and 1976, scoring 52 goals, and was a central figure in the club's Fourth Division promotion season in 1969-70.
He also represented Wales at Youth and Under-23 level.
Swans chairman, Huw Jenkins, said: "It's always sad to lose a member of the Swans family, especially when that person gave so much to the club as a player.
"Having played an incredible 357 league games for the club, there's no doubt he was an influential member of the Swans over a number of years.
"He was a well known and loved member of the local community going on to play in the Swansea League when he hung up his boots at the Vetch.
"Our condolences go out to his family and friends at this sad time."