Thailand entered the match with a 3-1 advantage from the first leg and relied on counterattacks as Singapore dominated possession early on.
The visitors created several opportunities but struggled to find a breakthrough, with Ilhan Fandi twice denied by Thailand goalkeeper Kampol Pathomakkakul.
Singapore finally scored in the 45th minute. After Pathomakkakul rushed out to clear a dangerous attack, the ball fell to Ryhan Stewart, whose shot from around 20 meters found the net to cut Thailand’s aggregate lead to 3-2.
Thailand responded after halftime and were awarded a penalty in the 60th minute when Wanchai Jarunongkran was brought down inside the box. Kakana Khamyok, who had come on at halftime, converted the penalty to make it 1-1 on the night and restore Thailand’s two-goal aggregate advantage at 4-2.
Singapore immediately responded. In the 65th minute, Stewart’s finishing attempt took a deflection off Thailand defender Pansa Hemviboon and looped into the net, reducing the aggregate deficit to 4-3.
The goal gave Singapore renewed hope, and the visitors pushed forward with additional attacking players.
Thailand, meanwhile, looked for chances on the counter, with Teerasak Poeiphimai hitting the post in the 69th minute.
Poeiphimai went close again later in the half, but his effort was saved by goalkeeper Izwan Mahbud before striking the near post.
Singapore continued pressing in the closing stages, but Thailand strengthened their defense and protected their one-goal aggregate advantage through five minutes of added time.
Thailand’s 4-3 aggregate victory sends them into the ASEAN Cup final. Their opponent will be decided by the second semifinal between Vietnam and Malaysia on Wednesday.