The Supreme Court upheld the ruling of two lower courts on Wednesday, sentencing red-shirt member Arisman Pongruangrong and 11 others to four years in prison over the riot that shut down the 2009 Asean Summit in Pattaya.
Another defendant, Somyos Promma, was acquitted. The court ruled he was only a protester taking part, not an organiser of the raid.
The ruling was read out in the Pattaya Provincial Court in Pattaya city, where the attack took place.
Thailand was the Asean chairman at the time and the summit was hastily cancelled due to the violent incursion by United Front for Democracy against Dictatorship members determined to oust then-prime minister Abhisit Vejjajiva.
In its ruling, the court found all 12 defendants guilty as charged for their role in the violent protest at the Royal Cliff Beach Resort Hotel, which forced the hasty evacuation of regional leaders and the postponement of the summit.
Sakda Noppasit, the deputy secretary-general of the Puea Chat Party, was only defendant at the court for the ruling. The court issued arrest warrants for the others to serve their jail terms.
One of the convicted defendants is Waipot Apornrat, a Palang Pracharath Party MP for Kamphaeng Phet province. Pol Lt Col Waipot was seen at parliament after the court read out the ruling but later left parliament house.
Deputy Prime Minister Wissanu Krea-ngam, the cabinet's legal expert, said after the sentencing that Waipot's MP status and political immunity ended with the final ruling because the court did not suspend any of the jail terms.
A by-election would be needed in constituency 2 of Kamphaeng Phet, he added.
The storming of the Asean Summit venue occurred on April 11, 2009, during the Abhisit Vejjajiva government.
The red-shirt protesters forced their way into the hotel, sparking concerns for the safety of the regional leaders. Some of them were evacuated by boat and some by helicopter.
Prosecutors filed suit against the group for defying an order prohibiting a rally of more than 10 people and violating traffic regulations.
The Pattaya Provincial Court, in 2015, found them guilty and sentenced each of them to four years in jail, without suspension. The Appeal Court upheld the ruling two years later.
The other nine who were convicted - in addition to Arisman, Sakda and Waipot - are: Nisit Sinthuprai, Payap Panket, Worachai Hema, Wanchana Kerddee, Pichet Sukjindathong, Nopporn Namchiangtai, Samrerng Prachamrua, Wallop Yangtrong and Singthong Buachum.
The disrupted Asean summit was later restaged in Phuket in June that year, with a strong military presence ensuring the proceedings were not disturbed a second time.