Ratchaburi defeated Buriram United 2-1 in the FA Cup semi-finals on Wednesday night to meet Port after they beat Bangkok United.
The result ended the Thunder Castle's hopes of a sweep of three domestic titles.
It was Buriram's second successive defeat after their loss to SCG Muang Thong United in Thai League 1 at the weekend.
Thai League 1 champions Buriram, who have reached the League Cup final, are now two points behind Chiang Rai United with four games remaining.
At Bangkok's 72nd Anniversary Stadium last night, Amin Nazari gave Ratchaburi the lead in the first half.
Pansa Hemviboon equalised for Buriram after 58 minutes only for Yannick Boli to score the winner in the 73rd minute.
Ratchaburi will meet Port in the final on Nov 2, as Port beat Bangkok United 5-4 on spot kicks after the two teams ended in a goalless draw during normal time.
Meanwhile, three clubs want to be a host venue for next year's AFC U23 Championship, the Football Association of Thailand (FAT) said on Wednesday.
FAT president Somyot Poompunmuang yesterday held a meeting with parties concerned to find a venue to replace Chiang Mai 700th Anniversary Stadium, which is under renovation and will not be ready for the tournament in January.
He said representatives of Buriram United, SCG Muang Thong United and BG Pathum United had shown interest to be a host of the tournament.
They have been asked to submit details of their stadiums and the FAT will make a decision later, Somyot said.
The other three venues for the tournament are Bangkok's Rajamangala National Stadium, Thammasat Stadium in Rangsit and Tinsulanonda Stadium in Songkhla.
A team from the Asian Football Confederation will inspect the stadiums from Sept 20-24 and the draw for the championship will be held on Sept 26.