Suttinun Phukhom's goal in the 88th minute gave the Sharks a crucial win at Chonburi Stadium.
With the victory, Chonburi moved to within one point of the Thunder Castle.
Third-placed SCG Muang Thong United also closed the gap on Buriram after a comfortable 3-0 home win against bottom club Samut Songkhram.
After 29 matches, defending champions Buriram have 59 points, Chonburi 58 and Muang Thong 57.
In an entertaining match at Chonburi Stadium, both sides squandered chances before Suttinun's header beat Buriram goalkeeper Sivarak Terdsungnern two minutes from time.
At SCG Stadium, Muang Thong had few problems against Samut Songkhram in a one-sided match.
Mario Gjurovski (35), Aritz Borda (69) and Cleiton Silva (90) were on target for the Kirins.
At Army Stadium, Army beat Chiang Rai United 3-1.
A own goal by Renan Marques gave the hosts the lead but Army's Seegate Mardputae sent the ball into his own net for 1-1 at half-time. In the second half, Anuwat Noichuenphan regained the lead for Army and Nipol Kumthong made it 3-1.
At Rajamangala National Stadium, hosts Osotspa drew 1-1 with Bangkok Glass. Blazhe Ilijoski opened the scoring for the visitors after four minutes only for Aron da Silva to equalise two minutes later.