The Singapore Sports Hub was packed with the local Singaporeans crowds, along with a few hundred Thais who had turned out to support their national side.
Singapore attacked from the outset but Praweenawat Boonyong and Adisorn Promrak worked hard to stem the tide.
However, Thailand drew first blood against the flow of play when Prakit Deeprom crossed the ball from the left side and Mongkol Thosakrai converted from close range.
Thailand could have gone further ahead in the 17th minute when Kirati Kaewsombut latched on to a cross but Singapore’s goalkeeper Hassan Sunny made the save.
Singapore’s equaliser came shortly after, when Khairul Amri found space and headed home.
Both sides squandered further chances in the first half before turning with the scores still level at 1-1.
After Singaporean captain Shahril Ishak missed an inviting chance, Thailand worked their way on top. But it was not until the final minutes of the match that they secured the decisive penalty after a Singaporean defender committed a hand-ball in the penalty box.
Charyl Chappuis converted from the spot to put his side ahead and secure an important opening victory.
“We were lucky to take the three points from this match," said Thai coach Kiatisak Senamuang. "I expected only one point before the match.
“We knew that Singapore would put a lot of pressure on our team and they counter-attacked well but our defence was also strong. This is football."
Earlier, underdogs Myanmar held 10-man Malaysia to a goalless draw in a bad-tempered opening match which was delayed by more than hour by heavy rain and thunderstorms.
Malaysia midfielder Gary Steven Robbat was sent off for a second bookable offence in the 34th minute as Myanmar controlled possession but could not find the goal which would have given them a first win in the biennial tournament since 2008. Bangkok Post