He died of cancer at his home in Vienna, Austria, the local media reported.
Since the beginning of this year his health was deteriorating, and he was cared for at home by his wife.
He started playing the game with ATSV Teesdorf, and later won two Austrian league titles with Austria Wien.
He later represented Sint-Truiden and Royal Antwerp in Belgium, finishing as the top scorer in the 1971 and 1972 seasons. He also won four caps for Austria in the 1970s.
He retired in 1985 and began coaching four years later.
He managed the Austrian team for a year, before beginning a marathon coaching career that took him to seven countries.
He had three stints with the Vietnamese team (1998-2000, 2003-04 and 2005-07), leading them to the final of 1998 AFF Championship (Tiger Cup) and three silver medals in the Southeast Asian Games (SEA Games) in 1999, 2003 and 2005.
Before Portuguese coach Henrique Calisto helped Vietnam win the 2008 AFF Cup, Riedl was considered the country’s most successful foreign coach.
Arguably his most outstanding achievement was taking Vietnam to the quarter-finals of the 2007 Asian Cup.
He also led Sanna Khanh Hoa FC (2001) and Hai Phong FC (2008-09).
Though he did not manage to win any titles with Vietnam, he received a lot of love from fans.
An unidentified fan even donated a kidney to him 13 years ago.
He was generally well respected in Asia, especially Southeast Asia, where he also coached and served as technical director of Laos and managed the Indonesian team on three separate occasions.
Last year the Football Association of Indonesia again offered Riedl the coach’s job, but he declined due to health reasons.