Dusit Chalermsan on Thursday became the second Thai League 1 coach to quit his job, his decision coming as a big "shock" to Port team chairwoman Nualphan Lamsam.
Former Thailand left wing back Dusit announced his resignation after his team fell way short of the club's pre-season expectations.
Dusit's decision came hours before Port's home game against Ratchaburi on Thursday night and in the wake of a stunning 3-1 defeat at the hands of Nakhon Ratchasima on Sunday.
He told media that he felt responsible for Port's below-par performance in the top-flight this season which left the team reeling in eighth place with just 15 points -- 11 behind co-leaders Bangkok United and Buriram United -- after 11 matches.
"I feel responsible for the team's failure to meet its early targets," said Dusit, who took BG Pathum United to the league title last season.
"I have to show responsibility and let the club find a more suitable person to guide the team to better results.
"I apologise to everyone that I couldn't deliver the expected results."
Nualphan, who is also the manager of the national senior and U23 teams and is better known to the Thai football fans as "Madam Pang", said: "The news of Coach Ong's [Dusit] decision to resign came as a shock.
"I have just returned from the Asian U23 Championship qualifying event in Mongolia and had let the staff coaches handle the club affairs.
"He put in a lot of hard work but we have to move on from here and make every effort possible to bring success to the club."
Port have appointed Sarawut Treephan, the technical advisor of the team, as the interim coach.
Meanwhile, Ibson Melo and Jakkit Phalapol struck in the second half to give promoted Khon Kaen United a 2-0 victory at Chiangmai United on Thursday night.
Khon Kaen goalkeeper Teerat Nakchamnan, who came in as a first-half replacement, was sent off in the five-minute final injury time.
In Wednesday night's late games, hosts Suphanburi suffered their sixth straight defeat, going down 1-0 to 10-man Chiang Rai United.
Brazilian defender Brinner Henrique sealed the win for the Beetles in the 84th minute, half an hour after losing Felipe da Silva to a red card for a high tackle on Suphanburi's Suphan Thongsong.
Also on Wednesday night, Samut Prakan City were held to a goalless draw at home by Muang Thong United.