The Jeju District Court made the landmark decision in a retrial involving 18 survivors, including 99-year-old Lim Chang-eui.
The ruling essentially quashes the convictions handed down by a military court in 1949, as it means the court is revoking the initial indictments on grounds of procedural errors in the court proceedings and that there are insufficient grounds to try the defendants.
The Jeju massacre was a government-civilian conflict that lasted from 1948-1954 and was an outgrowth of Korea's ideological division following its 1945 liberation from Japan's colonial rule.
A 2003 government report put the number of civilian deaths at between 25,000 and 30,000, about 10 percent of the island's population at the time.
In the decades after 1954, authoritarian governments distorted or covered up the truth behind the incident and muzzled victims for the sake of public order and peace, the victims and their families have claimed. Some rightists defended the government's bloody crackdown on islanders as part of a campaign to "exterminate communist sympathizers."
The victims applied for a retrial in 2017, claiming they were wrongfully convicted and served time in prison after being tortured and abused.
Liberal President Moon Jae-in issued an apology to the victims last year, vowing to provide them with every support and to retrieve the remains of the missing. Moon also became the first sitting president to attend the memorial service on the island after President Roh Moo-hyun did so in 2006. (Yonhap)