Breast and endometrial cancer are prevalent and share both hormonal and environmental risk factors. This study aimed to identify shared germline genetic risk loci for these cancers. In total, 1116 endometrial cancer cases, 3200 breast cancer cases, and 5021 healthy controls were included in a merged sliding window haplotype genome-wide association study (GWAS). This analysis employed a logistic regression model in PLINK v1.07. The results from this merged analysis were compared with previous individual analyses of the same samples. The analysis identified three loci that influenced both the risk of breast and endometrial cancer: 8p21.1 (OR 2.1; p 1.6 × 10⁻⁸), 16q24.3 (OR 2.4; p 3.8 × 10⁻⁸) and 17q11.2 (OR 1.3; p 4.3 × 10⁻⁸). This combined haplotype GWAS of endometrial and breast cancers identified three loci associated with shared genetic risk, two of which were novel: 16q24.3 and 17q11.2. Further studies are warranted to replicate these findings and to determine its pathophysiological role and future clinical implications.