Understanding the functional role of mutated genes in cancer is required to translate the findings of cancer genomics into therapeutic improvement. BTG1 is recurrently mutated in the MCD/C5 subtype of diffuse large B cell lymphoma (DLBCL), which is associated with extranodal dissemination. There, we provide evidence that Btg1 knock-out accelerates the development of a lethal lymphoproliferative disease driven by Bcl2 overexpression. We further show that the scaffolding protein BCAR1 is a BTG1 partner. Furthermore, following BTG1 deletion or expression of BTG1 mutations observed in DLBCL patients, the overactivation of the BCAR1-RAC1 pathway confers increased migration ability in vitro and in vivo. These modifications are targetable with the SRC inhibitor dasatinib, which opens novel therapeutic opportunities in BTG1 mutated DLBCL.