The acorn barnacle is a sessile crustacean, inhabiting the intertidal areas of tropical and temperate regions worldwide. According to current practices on Cirripedia morphology, shell, opercular valves, and arthropodal characters including cirri and mouthparts are used as a tool for taxonomic classification, and using these characteristics the present study aimed to provide better resolution for the barnacle diversity and geographical distribution within coastlines of Thailand: the Andaman Sea and the Gulf of Thailand. A total of ten species belonging to three families (Chthamalidae, Tetraclitidae, and Balanidae) were identified in this study. Subsequently, five species were newly recorded for the first time from Thailand’s coasts: Newmanella spinosus Chan & Cheang, 2016, Euraphia hembeli Conrad, 1837, Euraphia depressa (Poli, 1795), Tetraclita kuroshioensis Chan, Tsang & Chu, 2007, and Tetraclita singaporensis Chan, Tsang & Chu, 2007. The others, already mentioned in previous records, include: Tetraclita squamosa (Bruguière, 1789), Chthamalus malayensis Pilsbry, 1916, Amphibalanus amphitrite (Darwin, 1854), Amphibalanus reticulatus (Utinomi, 1967), and Megabalanus tintinnabulum (Linnaeus, 1758). Interestingly, acorn barnacles along the Andaman Sea occur abundantly, and are much higher in number of species (up to 8 species) than those found in the Gulf of Thailand’s coast (up to 6 species). This biased trend of species’ preferences is possibly due to the differences in oceanographic nature between two coastlines and the history of barnacle colonization.