Bud dormancy is an important developmental stage affecting blooming date and leafing date (LD) in Japanese apricot (Prunus mume), but the genetic factors controlling the chilling requirement (CR) and heat requirement (HR) for dormancy release and bud burst time remain to be elucidated. Here, a quantitative trait locus (QTL) analysis using two F1 segregating populations was conducted to identify loci affecting these traits. The genotyping-by-sequencing technique was used to construct two high-density genetic maps, one for NKSC, a population derived from high-chill ‘Nanko’ crossed with low-chill ‘SC’, covering 660.2 cM with 408 markers, and one for NINK, a population derived from low-chill ‘Ellching’ crossed with ‘Nanko’, covering 1314.2 cM with 718 markers. We observed four traits: CR and HR for dormancy release, blooming date, and LD over several years. To identify the QTL controlling the downregulation of DORMANCY-ASSOCIATED MADS-box6 (PmDAM6) in January’s leaf buds, in which PmDAM6 could act as a dose-dependent inhibitor of bud break, its transcript levels in leaf buds were determined. All traits segregated in the analyzed seasons in both populations. For leaf bud dormancy, CR and LD were highly correlated across years and traits in the NKSC population, while HR, LD, and PmDAM6 expression were highly correlated in the NINK population. The QTL analyses localized the significant QTLs controlling leaf bud CR and HR, LD, and PmDAM6 expression in leaf buds to a region in linkage group 4, which suggests that this locus controls dormancy release, bud break, and PmDAM6’s downregulation in Japanese apricot leaf buds.