TGFBR16A (rs1466445) is a common low penetrance tumor susceptibility allele with a minor allelic frequency of 0.051. Several meta-analyses show that TGFBR16A is associated with breast cancer risk with an OR ranging from 1.15 to 1.30. We developed a novel knockin mouse model of TGFBR16A to study mammary cancer development and progression in MMTVneu mice. Here we show that TGFBR16A accelerates tumor onset, increases the percentage of mice with tumors, and the percentage of mice with multiple tumors. Age at tumor onset, tumor formation, and tumor multiplicity depend on TGFBR16A allelic dosage. Functional characterization shows that TGFBR16A confers higher tumor growth and clonogenicity and enhances G0/G1 cell cycle progression through downregulation of p27. These findings establish TGFBR1*6A as a potent modifier of breast cancer development and progression, which may affect more than 10% of patients with breast cancer.