Angiotensin II is involved in tumor growth; however, the precise mechanism is not known. Platelets also contribute to tumor growth, and angiotensin II type 1 receptor (AT1) is expressed on the platelet surface. We hypothesized that interaction of platelets with tumor cells through AT1 receptor signaling promotes tumor metastasis. B16F1 melanoma cells were intravenously injected into Agtr1a knockout mice (AT1a(-/-)) and wild-type littermates (WT); the AT1a(-/-) mice exhibited a reduction in lung colonies. Angiotensin II induced expression of P-selectin on platelets in WT but not in AT1a(-/-) mice. A selective P-selectin neutralizing antibody decreased lung colony numbers in WT but not in AT1a(-/-) mice. Levels of vascular endothelial growth factor (VEGF) and stromal cell-derived factor 1 (SDF-1) receptor in platelets at metastatic locus were lower in AT1a(-/-) mice. Treatment of neutralizing antibodies against VEGF and CXCR4 decreased lung colony numbers in WT but not in AT1a(-/-) mice. In AT1a(-/-) mice, and both mobilization of progenitor cells expressing CXCR4(+)VEGFR1(+) cells from bone marrow and their recruitment to lung tissues were suppressed. These results suggest that AT1A signaling plays a critical role in tumor metastasis through P-selectin-mediated interactions of platelets with tumor and endothelial cells and through the AT1A signaling-dependent production of VEGF and SDF-1, which may be involved in mobilization of CXCR4(+)VEGFR1(+) cells.