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Summary Constitutive NF-κB signaling promotes survival in multiple myeloma (MM) and other cancers; however, current NF-κB-targeting strategies lack cancer cell specificity. Here, we identify the interaction between the NF-κB-regulated antiapoptotic factor GADD45β and the JNK kinase MKK7 as a therapeutic target in MM. Using a drug-discovery strategy, we developed DTP3, a D-tripeptide, which disrupts the GADD45β/MKK7 complex, kills MM cells effectively, and, importantly, lacks toxicity to normal cells. DTP3 has similar anticancer potency to the clinical standard, bortezomib, but more than 100-fold higher cancer cell specificity in vitro. Notably, DTP3 ablates myeloma xenografts in mice with no apparent side effects at the effective doses. Hence, cancer-selective targeting of the NF-κB pathway is possible and, at least for myeloma patients, promises a profound benefit., Highlights • GADD45β is a critical mediator of the NF-κB antiapoptotic function in MM • GADD45β binds to MKK7 and promotes MM cell survival by blocking MKK7/JNK signaling • GADD45β/MKK7 inhibitors display potent activity against MM, in vitro and in vivo • GADD45β/MKK7 inhibitors are far more cancer selective than IKK/NF-κB inhibitors, NF-κB is implicated in MM and other malignancies, but it is challenging to block NF-κB only in diseased cells. Tornatore et al. identify an interaction between MKK7 and NF-κB-regulated GADD45β as a therapeutic target and develop a peptide that disrupts the complex and selectively kills MM cells.