HER2-positive luminal B breast cancer (BC), a subset of the luminal B subtype, is ER-positive and HER2-positive BC which is approximately 10% of all BC. However, HER2-positive luminal B BC has received less attention and is less represented in previous molecular analyses than other subtypes. Hence, it is important to elucidate the molecular biology of HER2-positive luminal B BC to stratify patients in a way that allows them to receive their respective optimal treatment. We performed molecular profiling using targeted next-generation sequencing on 94 HER2-positive luminal B BC to identify its molecular characteristics. A total of 134 somatic nonsynonymous mutations, including 131 nonsynonymous single nucleotide variants and three coding insertions/deletions were identified in 30 genes of 75 samples. PIK3CA was most frequently mutated (38/94, 40.4%), followed by TP53 (31/94, 33.0%), and others were detected at lower frequencies. Recurrent germline mutations of MLH1 V384D were found in 13.8% (13/94), with a significantly high TP53 mutations rate. The frequency of MLH1 V384D germline mutation in individuals with HER2-positive luminal B BC was significantly higher than that observed in the controls. All 13 cases were classified as microsatellite stable tumors. Tumor mutation burdens (TMB) were not significantly different between MLH1 V384D carrier and wild type. The concordant results of microsatellite instability (MSI) and TMB suggest that the haploinsufficiency of MLH1 plays a role as a tumor predisposition factor rather than a direct oncogenic driver. Our study identified, for the first time, that MLH1 V384D germline variant is frequently detected in HER2-positive luminal B BC. MLH1 V384D germline variant may not only contribute to gastrointestinal cancer predisposition but may also contribute to BC in East Asians.