Trichoderma Pers. (Hypocreales, Sodariomycetes) is an ecologically and economically essential fungus due to its various biological activities, such as inhibiting the growth of phytopathogens in the soil and improving plant nutrient uptake. Moreover, it is well known that this fungus also has multiple trophic interactions with plants, mushrooms, and even animals. Trichoderma species are commonly found in forest soil inhabitants and are widespread. Moreover, more than 450 species are currently named to Trichoderma species worldwide. However, only 37 Trichoderma species are recognized in Korea by the National Institute of Biological Resources (https://species.nibr.go.kr/index.do). Thus, it is necessary to discover the diversity of Trichoderma species in Korea. Here, we sampled the forest soils from 11 mountains in Korea from 2013 to 2017, and 63 Trichoderma strains were isolated from the soil samples. Sanger sequencing was conducted from the isolates with translation elongation factor -1α (EF-1α) and RNA polymerase II subunit (RPB2) gene for molecular work and phylogenetic analysis. Morphological analysis was conducted to identify the culture and microscopic characteristics. As a result, we found candidates for nine novel Trichoderma species, six unrecorded Trichoderma species, and ten known Trichoderma species. This result highlights the existence of numerous undiscovered species awaiting exploration. Based on these findings, we propose that many Trichoderma species inhabit forest soils in Korea.