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As the global sea surface water temperature increases due to climate change, some zooxanthellate species have extended their habitat range to higher latitudes. <i>Palythoa</i> species, a colonial zooxanthellate cnidarian, is one such example of a range-extending animal. Recently, massive colonies of zoantharians that appeared to be of <i>Palythoa</i> spp. were discovered in a subtidal area of southern Jeju Island. Because a zoantharian-dominated ecosystem could indicate an unhealthy status, the documentation of its occurrence and species identification are crucial for subsequent studies. In this study, we report and confirm the presence of massive <i>Palythoa</i> colonies in Taeheung and Topyeong, off the southern coast of Jeju Island, using in situ underwater images and identify the species by sequencing the internal transcribed spacer rDNA. The resulting Bayesian inference tree clearly demonstrates that the massive colonies consist of <i>Palythoa mutuki</i> and are closely related to <i>P. mutuki</i> collected from the Ryukyu Archipelago in southern Japan. These records provide evidence of the northward expansion of subtropical and tropical marine organisms.