Neuropilin-1 (NRP-1), a non-tyrosine kinase receptor of vascular endothelial growth factor-165 (VEGF165), was found expressed on endothelial and some tumor cells. Since its overexpression is correlated with tumor angiogenesis and progression, the targeting of NRP-1 could be a potential anti-cancer strategy. To explore this hypothesis, we identified a peptide inhibiting the VEGF165 binding to NRP-1 and we tested whether it was able to inhibit tumor growth and angiogenesis. To prove the target of peptide action, we assessed its effects on binding of radiolabeled VEGF165 to recombinant receptors and to cultured cells expressing only VEGFR-2 (KDR) or NRP-1. Antiangiogenic activity of the peptide was tested in vitro in tubulogenesis assays and in vivo in nude mice xenotransplanted in fat-pad with breast cancer MDA-MB-231 cells. Tumor volumes, vascularity and proliferation indices were determined. The selected peptide, ATWLPPR, inhibited the VEGF165 binding to NRP-1 but not to tyrosine kinase receptors, VEGFR-1 (flt-1) and KDR; nor did it bind to heparin. It diminished the VEGF-induced human umbilical vein endothelial cell proliferation and tubular formation on Matrigel and in co-culture with fibroblasts. Administration of ATWLPPR to nude mice inhibited the growth of MDA-MB-231 xenografts, and reduced blood vessel density and endothelial cell area but did not alter the proliferation indices of the tumor. In conclusion, ATWLPPR, a previously identified KDR-interacting peptide, was shown to inhibit the VEGF165 interactions with NRP-1 but not with KDR and to decrease the tumor angiogenesis and growth, thus validating, in vivo, NRP-1 as a possible target for antiangiogenic and antitumor agents.