Wnt/β-catenin signaling is instrumental for the development of mammary gland and the properties of mammary stem cells (MaSCs). The Wnt signaling downstream effectors that engage in regulating MaSCs have not been extensively studied. Here, we report that Neuropilin-1 (Nrp1) expression is induced by Wnt/β-catenin signaling in MaSCs, and its function is critical for the activity of MaSCs. Nrp1 is particularly expressed in MaSCs that are marked by the expression of Protein C Receptor (Procr). Knockdown of Nrp1 by shRNA diminishes MaSCs' in vitro colony formation and in vivo mammary gland reconstitution ability. Similar results are seen when antagonizing Nrp1 using a dominant negative peptide. In genetic experiments, deletion of Nrp1 results in delay of mammary development. In addition, knockdown of Nrp1 inhibits MMTV-Wnt1 tumor growth in xenograft. Our data demonstrate that Nrp1 is critical for mammary development and tumorigenesis, revealing new insights into MaSC regulation and targeting stem cells in treatment of breast cancer.