A ngiogenesis, or new blood vessel formation, is critical for the growth and spread of tumors. Multiple phases of this process, namely, migration, proliferation, morphogenesis, and vascular stabilization, are needed for optimal tumor growth beyond a diffusion-limited size. The sphingosine 1–phosphate (S1P) receptor-1 (S1P₁) is required for stabilization of nascent blood vessels during embryonic development. Here we show that S1P₁ expression is strongly induced in tumor vessels. We developed a multiplex RNA interference technique to downregulate S1P₁ in mice. The small interfering RNA (siRNA) for S1P₁ specifically silenced the cognate transcript in endothelial cells and inhibited endothelial cell migration in vitro and the growth of neovessels into subcutaneous implants of Matrigel in vivo. Local injection of S1P₁ siRNA, but not a negative control siRNA, into established tumors inhibited the expression of S1P₁ polypeptide on neovessels while concomitantly suppressing vascular stabilization and angiogenesis, which resulted in dramatic suppression of tumor growth in vivo. These data suggest that S1P₁ is a critical component of the tumor angiogenic response and argue for the utility of siRNA technology in antiangiogenic therapeutics.