Apple decline symptoms were frequently observed on cv. Fuji apple orchards located in Gyeonggi, Gyeongbuk, and Gangwon provinces during surveys conducted from May until the end of September 2020. Three fungal strains were isolated from the margins of internal lesions of diseased apple trees, and their morphological characteristics were considered similar to Botryosphaeria sinensis. Phylogenetic analysis using internal transcribed spacer (ITS) regions, translation elongation factor 1-alpha (tef1), beta-tubulin (tub2), and the second largest subunit of RNA polymerase II (rpb2) gene sequences confirmed the closest relationship of isolates with B. sinensis at the species level. According to a pathogenicity test, the appearance of dark-brown discolorations and vascular necrosis on apple branches inoculated with the isolated strain KNUF-20-014 was observed. To the best of our knowledge, this is the first report of B. sinensis as the causal agent of apple disease in Korea.