Repeated low-dose γ-irradiation (IR) induces thymic lymphoma in mice because of oncogenic mutations propagating from a primitive hematopoietic stem/progenitor cell (HSC) in the bone marrow. It is well known that IR-induced thymic lymphomagenesis is markedly enhanced by p53 deficiency, yet data also indicate that p53-dependent apoptosis can actively drive tumor formation in this model. The latter was recently expounded on by findings from Puma-deficient mice, indicating that loss of this proapoptotic p53 target gene results in protection from IR-induced lymphomagenesis rather than enhanced susceptibility to. Similar to Puma, the transcription factor interferon regulatory factor 5 (Irf5) has been reported as a p53 target gene and is required for DNA damage-induced apoptosis. To date, no studies have been performed to elucidate the in vivo role of IRF5 in tumorigenesis. Given its essential role in DNA damage-induced apoptosis, we explored the tumor suppressor function of IRF5 in IR-induced thymic lymphomagenesis. Somewhat surprisingly, we found that thymic lymphoma development was significantly suppressed in Irf5(-/-) mice as compared with wild-type littermates. Suppression was due, in part, to reduced thymocyte and HSC apoptosis, resulting in reduced compensatory proliferation, and reduced replication stress-associated DNA damage. The observed effects were independent of p53 or Puma as these proteins were upregulated in Irf5(-/-) mice in response to IR. This study demonstrates an important new role for IRF5 in maintaining HSC homeostasis after IR and supports the non-redundant functions of IRF5, p53 and PUMA in DNA damage-induced lymphomagenesis. We propose that IRF5 may be an attractive target for developing therapeutic agents to ameliorate radiation-induced bone marrow injury.