S1P₂ sphingosine 1-phosphate receptor signaling can regulate proliferation, survival, morphology, and migration in many cell types in vitro. Here, we report that S1P₂^−/− mice develop clonal B-cell lymphomas with age, such that approximately half of the animals display this neoplasm by 1.5 to 2 years of age. Histologic, immunophenotypic, and molecular analyses revealed a uniform tumor phenotype with features of germinal center (GC)–derived diffuse large B-cell lymphoma (DLBCL). Tumor formation was preceded by increases in GC B cells and CD69⁺ T cells, as well as an increased formation of spontaneous GCs, suggesting that S1P₂ loss may promote lymphomagenesis in part by disrupting GC B-cells homeostasis. With the sole exception of rare lung tumors, the effect of S1P₂ gene disruption is remarkably restricted to DLBCL. In humans, 28 of 106 (26%) DLBCL samples were found to harbor multiple somatic mutations in the 5′ sequences of the S1P₂ gene. Mutations displayed features resembling those generated by the IgV-associated somatic hypermutation mechanism, but were not detected at significant levels in normal GC B cells, indicating a tumor-associated aberrant function. Collectively, our data suggest that S1P₂ signaling may play a critical role in suppressing DLBCL formation in vivo. The high incidence of DLBCL in S1P₂^−/− mice, its onset at old age, and the relative lack of other neoplasms identify these mice as a novel, and potentially valuable, model for this highly prevalent and aggressive human malignancy. [Cancer Res 2009;69(22):8686–92]