Masahiro Wada's men came into the day's action two points behind Buriram, but are now one point clear with three games remaining after their 1-0 win at Osotspa Saraburi and Buriram's 2-0 defeat at Bangkok United.
Chonburi were grateful to Kazuto Kushida's fifth-minute goal for their victory at Osotspa, which put pressure on Buriram as they took on Bangkok later in the day.
The previous leaders were unable to respond, however, as Romain Gasmi's brace condemned Chonburi to a fifth league defeat of the season, leaving them one point adrift.
At the opposite end of the table, the relegation of Air Force Central and Songkhla United was confirmed after both fell to defeat.
Air Force Central were edged out in a 5-4 thriller at Sisaket, who led 4-2 after an hour thanks in no small part to Brent McGrath's hat-trick.
The visitors looked to have earned a point when goals from Phuwanart Sangsri and Isaac Honny levelled matters, but Chatchai Mokkasem's last-gasp winner gave Sisaket's own survival chances a boost and ended the hopes of their opponents.
Songkhla are also down after they lost 2-1 at home to fellow strugglers PTT Rayong courtesy of goals from Prathan Mansiri and Narongchai Vachibaran, either side of Kayne Vincent's equaliser.
That result continued PTT's strong recent run of form, as Phayong Khunnaen's side made it six wins and two draws from their last eight outings, although they remain in 16th - the final relegation slot.
Joining Air Force Central and Songkhla in Division One will be already relegated Samut Songkhram, who lost for the fifth match in succession, 2-1 at home to TOT.
That proved a much-needed victory for TOT, who remain two points clear of danger in 15th, with PTT and Police United hot on their trail.
The latter earned their first win in seven with a thrilling 4-3 victory at home to Chiangrai United. After blowing a 3-1 lead, the hosts were grateful to Sergio Suarez's winner 18 minutes from time for securing all three points.
Muang Thong United moved to within three points of an AFC Champions League qualifying berth as they came from behind to defeat Army United 2-1, while BEC Tero Sasana remain in with an outside chance following a 3-1 success at Singhtarua.
Elsewhere, Suphanburi defeated Chainat Hornbill 1-0 and Ratchaburi prevailed 3-2 at home to Bangkok Glass.
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