Nongbua Pitchaya became the third team to be relegated after losing to Port in Thai League 1 action on Sunday night.
A 2-1 loss at their Pitchaya Stadium condemned the home team to Thai League 2 next season.
Nongbua headed into the game needing a win to keep their survival hopes alive and got off to a great start with a goal from Thanawut Phochai in the 32nd minute.
Port, however, hit back in the second half and got two goals in the space of just three minutes from Teerasak Poeiphimai (57th) and Bordin Phala (59th).
Nongbua are 14th with 27 points with one game remaining. Port moved up to fifth with 45 points.
The other two teams who have already been relegated are Nakhon Pathom and Khon Kaen United.
"It was a bad result. We wasted too many scoring opportunities and couldn't get that second goal. Then the momentum shifted and we lost concentration and got punished," said Nongbua coach Sukrit Yothee.
Nongbua will play their final game of the season on April 30 against leaders Buriram United at Chang Arena.
Meanwhile, Rayong avoided relegation after thrashing nine-man Khon Kaen 4-0 at WHA Rayong Stadium.
Bruno Cantanhede put the home team ahead in the 22nd minute before Stenio Junior made it 2-0 in the 39th.
Khon Kaen were reduced to 10 men when skipper Thawin Butsombut swung at Amani Aguinaldo and was red-carded after a VAR check in the 56th minute.
The visitors had another player sent off a few minutes later when Nopphol Ponkam went in with a nasty tackle on Jeon Hae-Min.
Aguinaldo got the third goal for Rayong in the 77 minute and Arthit Boodjinda made it 4-0 in the third minute of stoppage-time.
Rayong moved up to 12th spot with 32 points. The players were reportedly rewarded with a one-million-baht bonus from club chairman Sathit Pitutecha.
In other games on Sunday night, PT Prachuap held Nakhon Ratchasima to a 1-1 stalemate while Uthai Thani and Muang Thong United also drew 1-1.