Following their earlier 3-2 defeat to Korea Republic's Under-23 side, Tore Reginiussen's goal from close range seven minutes from time at Bangkok's Rajamangala Stadium, condemned the Thais to their second defeat in two games.
"We played better than in the Korea Republic match. We had chances but could not score," Thailand coach Winfried Schaefer told reporters after the match.
"We conceded the goal from a free-kick. I had told my players to be careful about free-kicks but this still happened."
Meanwhile, Korea Republic lead the King's Cup standing following a 0-0 draw with Denmark.
In the meantime, South Korea lead the standings in the 41st King's Cup after holding defending champions Denmark to a scoreless draw in the other game of the day.
The competition concludes on Saturday, with the Koreans only needing a point from their clash with Norway to lift the title.
Thailand play Denmark in the other match.