Breast cancer (BC) is the most common malignant disease in women worldwide. Several studies have reported that microRNA-146a (miR-146a) dysregulation plays a role in multiple cancers, including BC. However, the mechanism underlying this association is controversial, possibly reflecting diverse roles for this miR in different types of cancer. The SNP rs2910164:G>C, located within the miR-146a precursor, has been linked to a BC risk. Our group previously showed a specific association between rs2910164:G>C and an increased BC risk in patients with early-onset sporadic BC. There are no studies in the literature that evaluate the functional consequences of the rs2910164 polymorphism in the BC process. Therefore, the goal of the present study was to evaluate in vitro the effect of the SNP rs2910164:G>C on BC progression in luminal A and triple-negative cell lines. We found that rs2910164:G>C upregulated the expression of two mature miR-146a sequences, 3p and 5p. Furthermore, pre-miR-146a-C enhanced proliferation, migration, and invasion in luminal A and triple-negative breast cells, as well as decreasing cisplatin-induced apoptosis. Interestingly, the pre-miR-146a C allele decreased cisplatin resistance in MCF-7 cells but increased cisplatin resistance in MDA-MB-231 cells. We propose that the rs2910164 C allele promotes miR-146a overexpression, which is causally involved in proliferation, migration, invasion, apoptosis, and cisplatin resistance.