Collybiopsis is a genus of the gymnopoid/marasmioid complex of the family Omphalotaceae. The classification system of Collybiopsis has recently undergone large changes through molecular approaches. The new classification system has not been applied for Collybiopsis in the Republic of Korea, and general research on this genus was also lacking. In this study, we analyzed the Collybiopsis species in the Republic of Korea by assessing all gymnopoid/marasmioid specimens collected nationwide for ten years by combining morphological approaches and multilocus (ITS + nrLSU) phylogenetic analysis. We thus confirmed that 16 species of Collybiopsis are present in the Republic of Korea, including two previously unreported species (Co. nonnulla and Co. dichroa) and seven new species (Co. albicantipes sp. nov., Co. clavicystidiata sp. nov., Co. fulva sp. nov., Co. orientisubnuda sp. nov., Co. subumbilicata sp. nov., Co. undulata sp. nov., and Co. vellerea sp. nov.). A thorough examination of the Collybiopsis suggested that it is difficult to distinguish or identify the species based on morphological characteristics only; a combined molecular approach is needed for accurate identification. The Collybiopsis database of the Republic of Korea is updated, and information on the new species is provided. Five new combinations from Marasmiellus to Collybiopsis are also proposed (Co. istanbulensis comb. nov., Co. koreana comb. nov., Co. omphalodes comb. nov., Co. pseudomphalodes comb. nov., and Co. ramuliciola comb. nov.).