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.
The 1998 Vietnam team under Riedl, including defender Tran Cong Minh, forward Le Huynh Duc, and midfielders Nguyen Huu Thang and Nguyen Hong Son, is often looked up to as the golden age of Vietnamese football.
He is loved and respected by generations of players, who still call him "teacher."
Former midfielder Phung Thanh Phuong, who played at the 1998 AFF, described Riedl as an "exemplary" person who brought positive changes to Vietnam football's playing style and way of thinking.
"He's caring, calm and fair. I feel really painful knowing that he died."
Nguyen The Anh, Vietnam's main goalkeeper at SEA Games 2003, still remembers Riedl's exact words to boost their morale when the team lost to Thailand in the tournament's final: "Today everyone's sad, every Vietnamese person is sad, but you must not be discouraged, because that's how football works. You are young and your future is still ahead. You should stand up from this fall."
"Coach Riedl is a very devoted and responsible person," Anh said.
Star forward Le Cong Vinh, who played with Riedl for four years, said he still holds many memories with his coach, and his death was a shock to him.
"He's a coach that I, and probably many other players, respect both personally and professionally.
"He's gone but all the things that he has done for Vietnam football will live."
Riedl was considered the country’s most successful foreign coach before Portuguese coach Henrique Calisto helped Vietnam win the 2008 AFF Cup, where Vinh played a key role.
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.