The Football Association of Thailand confirmed yesterday that only two teams were relegated from the top flight to the second-tier league.
At the end of the 2019 Thai League 1 season on Saturday, Chiangmai, Chainat and Suphanburi were the last three teams.
Normally, the bottom three clubs go down to Thai League 2 at the end of the season.
However, executives of PTT Rayong, who finished 11th, announced on Monday that they decided to suspend the club indefinitely.
FAT secretary-general Korrawee Pritsananantakul said the rules clearly state that if a team withdraws from the league, only two clubs will be relegated.
"As a result, Suphanburi [who were third from last] will remain in the top league next season," said Korrawee, who is also deputy chairman of the league's organising committee.
On the other hand, he said, there will only be 17 teams in Thai League 2 next year because Suphanburi remain in Thai League 1.
BG Pathum United, Police Tero and Rayong FC have gained promotions from T2 to T1.
The T1 championship was decided on the final day of the season on Saturday.
Already-relegated Chiangmai held Buriram United to a 1-1 draw with a late equaliser.
The stalemate handed the title to Chiang Rai United, who beat Suphanburi 5-2.
Chiang Rai and Buriram both had 58 points but the Beetles had a better head-to-head record.
FAT president Somyot Poompunmuang said yesterday that Chiang Rai's first ever league crown was a good example for any team.
"Chiang Rai won the title with their determination and hard work," Somyot said.
"This should be a motivation for every team that you can be successful with determination and hard work."