The routine win saw Newin Chidchob's charges take an unassailable lead over Chonburi FC and Muang Thong United who are currently second and third respectively, and claim their first ever title with four league games remaining.
Anderson dos Santos broke the deadlock in the 30th minute for Buriram. Army United equalised three minutes later through Leandro dos Santos, but further first half goals from Ekwalla Herman and Franck Ohandza sealed Buriram's victory.
Buriram remain on track for a domestic treble, with a League Cup final against Thai Port and an FA Cup quarter-final still to come.
In the other weekend matches, Sri Racha drew 2-2 with Bangkok Glass, Pattaya United defeated Chiang Rai United 3-0 and TOT lost 3-2 to TTM Phichit.