Homologous recombination deficiency (HRD) tests, including MyChoice CDx, are companion diagnostics for poly (ADP-ribose) polymerase (PARP) inhibitors. BRCA1 promoter hypermethylation, a major HRD cause, may correlate with poorer prognosis. This study aimed to develop a simple, accurate method for detecting BRCA1 promoter hypermethylation and elucidate the characteristics of such cases. BRCA1 promoter methylation was analyzed using bisulfite sequencing (BIS-seq) in high-grade serous ovarian carcinoma specimens. We developed a newly developed BRCA1 methylation assay, BRCA1-Fragment Analysis of Methylation (BRCA1-FAM), which combines restriction enzyme digestion with fragment analysis. The accuracy of this assay was compared to the results of BIS-seq. We evaluated the relationship between BRCA1 promoter hypermethylation and prognosis and examined its association with BRCA1 expression and loss of heterozygosity. BRCA1 mutations and promoter methylation were mutually exclusive in the analyzed cases, with methylation observed in 28.9% (22/76) of primary debulking surgery cases. The BRCA1-FAM showed high sensitivity (91.3%) and specificity (100%) for detecting BRCA1 promoter hypermethylation, comparable to BIS-seq. Cases with BRCA1 promoter hypermethylation had significantly poorer progression-free survival (log-rank test, p = 0.048). Among these cases, 86.4% displayed abnormal BRCA1 immunostaining, with lower frequencies of BRCA1 loss of heterozygosity compared to those of other groups. BRCA1 promoter hypermethylation is associated with poor prognosis, underscoring the importance of its identification for HRD stratification. BRCA1-FAM is a simple and highly accurate method for evaluating BRCA1 promoter methylation. This approach may potentially enhance the precision of personalized therapies for ovarian cancer.