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NF-κB proteins play a central and subunit-specific role in the response to DNA damage. Previous work identified p50/NF-κB1 as being necessary for cytotoxicity in response to DNA alkylation damage. Given the importance of damage-induced cell death for the maintenance of genomic stability, we examined whether Nfkb1 acts as a tumor suppressor in the setting of alkylation damage. Hprt mutation analysis demonstrates that Nfkb1(-/-) cells accumulate more alkylator-induced, but not ionizing radiation (IR)-induced, mutations than similarly treated wild-type cells. Subsequent in vivo tumor induction studies reveal that following alkylator treatment, but not IR, Nfkb1(-/-) mice develop more lymphomas than similarly treated Nfkb1(+/+) animals. Heterozygous mice develop lymphomas at an intermediate rate and retain functional p50 in their tumors, indicating that Nfkb1 acts in a haploinsufficient manner. Analysis of human cancers, including therapy-related myeloid neoplasms, demonstrates that NFKB1 mRNA expression is downregulated compared with control samples in multiple hematological malignancies. These data indicate that Nfkb1 is a haploinsufficient, pathway-specific tumor suppressor that prevents the development of hematologic malignancy in the setting of alkylation damage.