Abstract Secreted frizzled-related protein 2 (SFRP2) is overexpressed in human angiosarcoma and breast cancer and stimulates angiogenesis via activation of the calcineurin/NFATc3 pathway. There are conflicting reports in the literature as to whether SFRP2 is an antagonist or agonist of β-catenin. The aims of these studies were to assess the effects of SFRP2 antagonism on tumor growth and Wnt-signaling and to evaluate whether SFRP2 is a viable therapeutic target. The antiangiogenic and antitumor properties of SFRP2 monoclonal antibody (mAb) were assessed using in vitro proliferation, migration, tube formation assays, and in vivo angiosarcoma and triple-negative breast cancer models. Wnt-signaling was assessed in endothelial and tumor cells treated with SFRP2 mAb using Western blotting. Pharmacokinetic and biodistribution data were generated in tumor-bearing and nontumor-bearing mice. SFRP2 mAb was shown to induce antitumor and antiangiogenic effects in vitro and inhibit activation of β-catenin and nuclear factor of activated T-cells c3 (NFATc3) in endothelial and tumor cells. Treatment of SVR angiosarcoma allografts in nude mice with the SFRP2 mAb decreased tumor volume by 58% compared with control (P = 0.004). Treatment of MDA-MB-231 breast carcinoma xenografts with SFRP2 mAb decreased tumor volume by 52% (P = 0.03) compared with control, whereas bevacizumab did not significantly reduce tumor volume. Pharmacokinetic studies show the antibody is long circulating in the blood and preferentially accumulates in SFRP2-positive tumors. In conclusion, antagonizing SFRP2 inhibits activation of β-catenin and NFATc3 in endothelial and tumor cells and is a novel therapeutic approach for inhibiting angiosarcoma and triple-negative breast cancer. Mol Cancer Ther; 12(5); 685–95. ©2013 AACR.