Thailand exited the AFC U20 Asian Cup in the group stage following a 4-1 loss to South Korea in Shenzhen, China, on Monday night.
The win assured South Korea a top-two finish irrespective of their final Group D clash against Japan, who drew 2-2 with Syria earlier on Monday.
The defeat eliminated Thailand, who lost 3-0 to Japan in their first match.
The 12-time champions South Korea kept pushing forward, but it was Thailand who scored in the 23rd minute when Yotsakon Burapha received a through ball from Pongsakorn Sangkasopha following a fast break.
The Koreans were unperturbed, and Yoon Do-Yong scrambled the ball home following a corner from Le Chang-Woo to level the score in the 32nd minute. This paved the way for goals from Kim Tae-Won (59, 86) and Park Seung-Soo (89).
South Korea coach Lee Chang-Won said after the match: "Our physique is superior to the Thais, so we tried to use that advantage in our set pieces and it worked well.
"In the second half, we brought in fast players and that strategy worked to our benefit as well.
"Thailand had to win after their loss against Japan and prepared well, especially in their attack from the flanks. They also defended well and we struggled but managed in the end."
Thailand U20 coach Emerson Pereira admitted his side lost power and focus after the break.
"We had a good opening half with our first goal and we had many chances to score," he said.
"But in the second half, we lost power, made mistakes, and allowed them to score three times.
"We need to analyse our game against South Korea, fix the problems and focus 100 percent because it's 95 minutes of football until the referee blows the whistle."
A win for Lamphun
Lamphun Warriors claimed three points from their Thai League 1 game at Rayong after a 3-2 victory on Monday night.
The away win helped Lamphun move six points clear of the three-team relegation zone which includes their opponents Rayong.
