Singapore took a two-goal advantage over Thailand and clung on despite near-relentless pressure from the spirited home side who were left to rue their dismal 1-3 first leg loss in the city-state on Wednesday.
Desperate to make amends Thailand's War Elephants threw everything at the visitors, with a bullet header from Kirati Keawsombut just before half-time giving them hope of securing a victory in front of fevered home support.
But they could not find a second goal to take the game into extra-time, with Singapore's defence resolute in the face of wave after wave of Thai attacks. It was an impressive third cup victory for their coach Radojko Avramovic, who guided Singapore to the 2004 and 2007 titles.
But he will inevitably face criticism for negative tactics despite a pre-match vow to go for the jugular in Thailand. In reality his side barely registered a shot in the Supachalasai Stadium.
"In the end we deserved to win, because we scored three goals," an unrepentant Avramovic said after the game, which will be his last in charge. It was a long, hard campaign. Thailand were not the losers today, we won at home and they won here, but the difference is that we had some luck."
Baihakki Khaizan, who scored the killer third goal in Singapore on Wednesday, starred for the visitors, making a series of key blocks and headers at the heart of the defence to keep the plucky Thai front line - including the prolific Teerasil Dangda - at bay.
He was helped by his goalkeeper Izwan Mahbud who made a vital save minutes from the end after substitute Chanathip Songkrasin slalomed into the box before unleashing a shot which would have taken the final into added time.
"They all gave 100 per cent but it was just not quite enough. I don't like to come to second but I'm proud of it," said Thailand's German coach Winfried Schafer. "This team was fantastic and everybody can see Thailand has enough talent."
Thailand, who have been runners-up three times in the last four years, probed throughout the first-half without making a breakthrough, hitting the bar and frequently finding space out wide to cross into the packed box.
Kirati's goal came after 15 minutes of pressure, powering in a header from a right side corner which sparked delirium in the stands as a giant Thai tricolore cascaded down one stand and smoke from flares wafted over the pitch.
But they could not find the second despite the jinking runs of Jakkapan Pornsai, or the pin-point crossing of Piyaphon Buntao from right midfield. Ultimately the Lions had their dominant first-leg performance to thank for their historic win, with goals from Mustafic Fahruddin, Khairul Amri and Khaizan leaving Thailand with too much to do.