Japanese apricot (Prunus mume) is a culturally and economically important fruit tree in East Asia. ‘Nanko’, the primary cultivar of the Japanese apricot well known in Japan, usually suffers from scab, a disease caused by Venturia carpophila. Scab results in the development of black spots on the fruit surface, which considerably reduces its commercial value. Despite being an economic concern for Japanese apricot, there have been few reports on the phenotypic variation in scab resistance/susceptibility, the underlying genetic factors, and the development of DNA markers. Therefore, in this study, we performed a comprehensive trait analysis for scab resistance and susceptibility for four years and conducted a genome-wide association study (GWAS) to identify the associated loci. The results showed that significant GWAS peaks were detected in 2017 and 2018. Promising candidate gene encoding domains associated with disease resistance were found at the nine single-nucleotide polymorphisms (SNPs) identified in 2017 and 2018. These SNPs were thought to be associated with scab susceptibility of ‘Nanko’ lineages. These findings shed light on the mechanisms of scab resistance in P. mume and will assist future breeding programs to improve scab resistance in ‘Nanko’ lineages.