Vasoactive intestinal peptide (VIP) receptor-2 (VIPR2) is a G protein-coupled receptor, and VIPR2 gene copy number is increased in breast cancer. We have found that VIPR2 forms a homo-dimer ligand-independently in breast cancer cell lines, thus we examined whether dimeric VIPR2 plays a role in the progression (promotion of growth and metastasis) of breast cancer. Reduction in VIPR2 expression or treatment with a VIPR2-selective antagonist peptide KS-133 suppressed VIP-induced breast cancer cell migration and proliferation. To identify the domains responsible for dimerization of VIPR2, we established several truncation mutants of VIPR2 and performed a binding assay with full-length VIPR2. Then we demonstrated the interaction of transmembrane-domain 3-4 (TM3-4) with VIPR2. FRET and pull-down assays revealed that VIPR2-VIPR2-interaction is suppressed in cells expressing TM3-4 peptides. Breast cancer cells stably expressing TM3-4 derived from VIPR2 exhibited suppression of proliferation and lymph-node metastasis in vivo. These results suggest that dimeric VIPR2 is involved in breast cancer growth and metastasis. Hence, VIPR2 represents a potential therapeutic target for protection against cancer progression. These findings would provide a rationale that KS-133 and TM3-4 peptides are candidates of novel molecularly targeted drugs.