The Cluster Munition Coalition (CMC) said it had investigated and verified Cambodian government claims that the deadly weapons had landed on its territory in four days of unrest between the neighbours in early February.
The group, which campaigns against the bombs, also said the Thai ambassador to the United Nations in Geneva had confirmed the use of cluster munitions in a meeting on Tuesday.
"The ambassador said Thailand used cluster munitions 'in self-defence', using the principles of 'necessity, proportionality and in compliance with the military code of conduct'," it said in a statement.
"It's appalling that any country would resort to using cluster munitions after the international community banned them," added CMC director Laura Cheeseman.
Thai officials were not immediately able to confirm or deny the ambassador comments.
Thailand's alleged cluster bomb use