The bulb onion (Allium cepa L.), one of the most important vegetables worldwide, contains various functional compounds such as quercetin, allicin, and flavonoids. Red onions are rich in anthocyanins, a flavonoid that is a functional phytochemical with antioxidative and anticancer activities. In the previous study, two quantitative trait loci (QTLs) (qAC4.1 and qAC4.2) controlling the anthocyanin content were identified on chromosome 4 in an F₂ population derived from a cross between A. cepa ‘SP3B’ and ‘H6’. In this study, we developed single nucleotide polymorphism-based high-resolution melting (HRM) markers linked to QTLs qAC4.1 and qAC4.2. In addition, we constructed a new genetic linkage map of chromosome 4 using HRM markers and performed a QTL analysis. The QTL qAC4.1 was false, while qAC4.2 was a major QTL. The QTL peak position, logarithm of the odds value, and phenotypic variance explained of qAC4.2 was 53.6 cM, 7.45, and 22.51%, respectively. Four HRM markers (AC4.2_65336.1_1123-HRM, AC4.2_53230.3_454-HRM, AC4.2_11999.1_756-HRM, and AC4.2_14596.1_345-HRM) within the QTL region of qAC4.2 were developed in this study. The average anthocyanin content of B (homozygous paternal) genotypes was higher than that of A (homozygous maternal) and H (heterozygous) genotypes for all markers. Consequently, these markers will be useful for marker-assisted selection to develop onion cultivars with high anthocyanin content.