Twenty-one species are reported for the first time from Koh Tao (Turtle Island) in the Gulf of Thailand. Information and photographs were obtained from local scuba divers in order to census rare (i.e., cryptic species or transient species only present during one season) or not previously recorded fish species living on or near coral reefs from the area. This is the first time that species belonging to the families Antennariidae (genera Antennatus and Histrio), Ophichthyidae (genus Callechelys), Platycephalidae (genus Thysanophrys), Plotosidae (genus Plotosus) and Synanceiidae (genera Inimicus and Synanceia), and reef-fish genera of several families (Pseudobalistes, Balistidae; Cyclichthys, Diodontidae; Bolbometopon, Scaridae; and Hippocampus, Syngnathidae), among others, have been recorded in Koh Tao. Of the 21 species reported for the first time from Koh Tao, 7 species (Antennatus nummifer (Antennariidae), Pseudobalistes marginatus (Balistidae), Monacanthus chinensis (Monacanthidae), Callechelys marmorata (Ophichthyidae), Thysnophrys cf. chiltonae (Platycephalidae), Bolbometopon muricatum (Scaridae) and Synanceia cf. verrucosa (Synanceidae)) are new records for the Gulf of Thailand. To date, 223 species of coral reef fishes belonging to 53 families are known from Koh Tao, including the species found in the present study.