Robson, capped 90 times by England, parted company with the team by mutual consent and preferred not to disclose his reasons, Football Association of Thailand President Worawi Makudi told Reuters.
"Bryan wished to resign and his contract was terminated by mutual consent but I cannot give any more details," Worawi said.
The former Middlesbrough, West Bromwich Albion, Sheffield United and Bradford City boss, dubbed "Captain Marvel" as a player, failed to make an impact in Thailand after replacing compatriot Peter Reid in 2009.
Under his guidance, the team were a shadow of the free-flowing side that Reid built and dropped 15 places to 120th in the FIFA rankings following embarrassing early exits from all international competitions.
THROAT CANCER
Robson underwent surgery for throat cancer in March and immediately resumed his coaching duties after being discharged from hospital. Worawi declined to comment when asked if Robson resigned for health reasons.
His departure comes amid strained relations between the Thailand and England Football Associations (FA), which helped recruit Reid and Robson for the Thai job.
In an address to Britain's parliament, former FA chairman David Triesman last month accused Worawi, a member of FIFA's Executive Committee, of asking for exclusive television rights for a proposed friendly between the two national teams in return for his backing for England's failed 2018 World Cup bid.
Worawi was cleared of wrongdoing by a FIFA ethics committee and threatened to sue Triesman for "groundless" accusations.
Robson is unlikely to be missed in Thailand following almost two years of dire performances which led to protests against the football association, with web boards inundated by comments by furious fans.
Thailand failed to reach the Asian Cup for the first time in 23 years and were bundled out of the Southeast Asian Games and the Southeast Asian championship - competitions they have long dominated - in the opening rounds.
Draws against struggling Laos, Malaysia and the Maldives last year turned up the pressure on Robson, but he refused to quit, saying the blame was both on him and the new-look Thailand Premier League, which he said was not co-operating with the national team.
