Fungi that infect mushrooms and cause diseases are called mycoparasitic fungi. Understanding the diversity of mycoparasitic fungi in wild mushrooms is important for the mushroom industry, as they can parasitize cultivated mushrooms. However, few studies have been conducted on the diversity of mycoparasitic fungi in wild mushrooms in South Korea. Upon surveying mushroom flora, we found a mycoparasitic fungus and analyzed its strain using molecular phylogeny and morphological examination. The strain was identified as Sepedonium laevigatum, which is an unrecorded species in South Korea. In this study, we described the cultural and microscopic characteristics of this strain of fungus.