The phylum Chaetognatha includes the Spadellidae, characterized by exclusive benthic species. Over the recent decades, 10 species belonging to Spadellidae have been discovered, each with unique habitats worldwide, including sediments, marine caves, and hydrothermal vent sites, underscoring the fragmented and isolated spatial distribution of Spadellidae species. Here, we report the first presence of Spadella japonica and Paraspadella gotoi in South Korean waters, two species previously described from specimens collected in South Japan. Comparative morphological analysis identified several variations from the former descriptions of S. japonica, mainly a larger adult size, a higher number of grasping spines, and the presence of posterior teeth. We also provide new scanning electron microscopy observations of P. gotoi which especialy focuses on the morphology of the adhesive organs. Molecular analyses using new sequences of small subunit ribosomal RNA genes enable to reassess the relationships of Spadellidae within Phragmophora with the broadest taxonomic sampling to date. We also generated cytochrome c oxidase subunit I mitochondrial (mtCOI) barcodes and all species always formed monophyletic groups in the phylogenetic analyses. Additionally, we observed highly divergent mitochondr...