Alejandro Menendez's side had not tasted defeat in Thailand's top flight since a 2-1 setback at Chiangrai United in October 2012.
Buriram were crowned Thai champions last season after going the full campaign undefeated, but were undone in just the third game of the new season at the 72-Years Anniversary Stadium on Saturday.
There was no sign of the visitors seeing their run end when Jay Simpson scored from the penalty spot after 13 minutes, yet the hosts led at the break thanks to a quickfire double from Apiwat Ngualamhin and Gilbert Koomson shortly before half-time.
Victor Leonel Altobelli levelled the scores in the 55th minute, but the home side snatched an unlikely victory through Peerapat Notchaiya on 61 minutes.
Muang Thong United remain top of the pile despite dropping points for the first time this season in a 1-1 draw at home to Police United.
Mario Gjurovski opened the scoring in just the fifth minute, but Surachart Sareepim equalised in the second half.
Chonburi failed to win for the first time this campaign in a thrilling 3-3 draw with Ratchaburi, though it could have been worse for the home side as it required a late double from Wanit Chaisan to secure the point.
Suphanburi chalked up their first win this term in 2-0 triumph over Bangkok Glass, while the matches between Chainat and Sisaket and Army United and TOT both finished 1-1.
On Sunday, Bangkok United moved off the foot of the table as goals from Ekkachai Sumrei and Paitoon Nontadee secured a 2-1 win at Osotspa.
Singhtarua improved to third as a Leandro Oliveira double helped overcome a losing position in a 2-1 win over Chiangrai United.
Also on Sunday, Samut Songkhram lost 1-0 at home to PTT Rayong, and Air Force Central drew 1-1 with Songkhla United.
