Chonburi, Police Tero and Trat were relegated with one round of matches remaining in the Thai League 1 on Saturday.
Ten-man Chonburi were hammered 6-0 by hosts Muang Thong United at Supachalasai Stadium.
The defeat condemned the Sharks, who were 15th on 27 points, to League 2 for the first time after 18 seasons in Thailand's highest league.
Trat had a 2-2 home draw with Lamphun Warriors and remained at the foot of the table with 26 points while Police Tero, who shocked second-placed Bangkok United 3-0 on Friday, were 14th with 28 points.
A point at Trat Stadium saw Lamphun move to 11th spot on 32 points.
Khon Kaen and Sukhothai, who were to face off last night, were 12th and 13th on 32 points.
A day after being crowned champions for the ninth time, Buriram United cruised to a comfortable 2-0 win at Uthai Thani 2-0.
The Thunder Castle moved to 66 points, eight clear of second-placed Bangkok United.
Chonburi's Chalermpong Kerdkaew was sent off for a foul on Poramet Arjvilai in the 29th minute after a VAR check.
Jaroensak Wonggorn (33rd minute) and Kakanak Khamyok (43rd), Willian Popp (54th, 72nd), Teerapol Yoryoei (79th) and Lee Jae-Sung (82nd) were on target in the Kirins' comfortable victory.
"It was a disappointing match for our team. Before the match, we thought that if we won the last two matches we would still have a chance to stay in the Thai League 1 but we could not do it. We tried our best but with one player less, it really made it difficult for us and the team could not find a way to come back into the game," said Chonburi team manager Sasin Singtothong.
"We are relegated to the second division and now we have to focus on the next season. We know our fans and the players are disappointed but we promise that we will come back in one year," added Sasin.
At Trat Stadium, Santiparb Ratniyom put the home team ahead on 20 minutes before Negueba equalised for Lamphun four minutes later. Mohammed Osman put Lamphun in front on 70 minutes but Jorge Fellipe's 81st-minute equaliser secured a point for Trat, who went straight back to League 2 after just one season in the top flight.
PT Prachuap, meanwhile, avoided a relegation after they defeated Chiang Rai United 1-0 at Sam Aoh Stadium.
The Killer Wasps climbed up to eighth spot with 34 points while Chiang Rai were ninth, with the same number of points.
Samuel Rosa scored the winning penalty in the 25th minute after a handball by Chiang Rai's Athibodi Atirat.
Second-half goals from Guilherme Bissoli (51st minute) and Supachai Chaided (58th) secured the points for the champions at Uthai Thani.
Buriram moved to 66 points, eight ahead of second-placed Bangkok United.
"It was not an easy game at all. But after we knew that we won the trophy, I told the players to keep playing well," Buriram coach Jorginho.