Fungal isolate ARI-25-A10 was obtained from an ambrosia beetle collected in an apple orchard in Korea. The isolate was identified through an integrative taxonomic framework incorporating cultural and morphological characterization, growth responses under different temperature and pH conditions, and multilocus phylogenetic analyses. When cultured on potato dextrose agar at 25°C for 5 days, colonies reached 65.3–67.3 mm in diameter. Colony morphology transitioned from white to pale smoke grey to light olivaceous as incubation progressed, and sclerotia developed during extended culture. In temperature-dependent growth assays, colony development was minimal at 10°C and 30°C, whereas optimal growth was observed between 15°C and 25°C. In pH-response assays, optimal growth occurred at pH 5, with stable growth also observed between pH 4 and 6, indicating an optimal pH range of 4–5. Hyphae were hyaline, irregularly septate, and averaged 2.2 μm in width. BLAST analyses of the internal transcribed spacer, actin, RNA polymerase II second largest subunit, translation elongation factor 1-α, and β-tubulin gene sequences revealed high sequence similarity between ARI-25-A10 and Cytospora ceratosperma. In addition, multilocus phylogenetic reconstruction using concatenated datasets placed ARI-25-A10 within a strongly supported clade comprising previously reported and characterized C. ceratosperma strains. This study represents the first report of C. ceratosperma associated with an ambrosia beetle collected from an apple orchard in Korea.