Rangsan Viwatchaichok resigned as the head coach of Port after the Thai League 1 giants fell 2-0 at Bangkok United on Saturday night.
The result dealt a heavy blow to Port's title aspirations as they now lag five points behind their opponents who lead the table with 27 points.
Bassel Jradi put the Angels ahead in the 11th minute before Oman striker Muhsen Al-Ghassani added the second goal on 72 minutes.
Rangsan announced his decision to quit at a post-match press conference.
"Actually, I was determined to speak about our recent results which to me have been nothing less than a failure," said Rangsan, adding: "We should have been in a much better position than we are now.
"Now, I have to assume the responsibility for this failure and quit as the head coach of Port.
"I would liked to thank the club for giving the chance with full authority. I think I am still not good enough to help Port realise their trophy dream."
Rangsan is the seventh Thai League 1 coach to lose his job this season.
In the other games on Saturday night, Muang Thong United lost 2-0 to PT Prachuap at home and hosts Ratchaburi were held to 2-2 draw by Sukhothai.