Traumatic brain injury (TBI) pathologies are caused by primary and secondary injuries. Primary injuries result from physical damage to the brain, and secondary injuries arise from cellular responses to primary injuries. A characteristic cellular response is sustained activation of inflammatory pathways commonly mediated by nuclear factor-κB (NF-κB) transcription factors. Using a Drosophila melanogaster TBI model, we previously found that the main proximal transcriptional response to primary injuries is triggered by activation of Toll and Imd innate immune response pathways that engage NF-κB factors Dif and Relish (Rel), respectively. Here, we found by mass spectrometry that Rel protein level increased in fly heads at 4-8 hr after TBI. To investigate the necessity of Rel for secondary injuries, we generated a null allele, Rel del , by CRISPR/Cas9 editing. When heterozygous but not homozygous, the Rel del mutation reduced mortality at 24 hr after TBI and increased the lifespan of injured flies. Additionally, the effect of heterozygosity for Rel del on mortality was modulated by genetic background and diet. To identify genes that facilitate effects of Rel del on TBI outcomes, we compared genome-wide mRNA expression profiles of uninjured and injured +/+, +/Rel del , and Rel del /Rel del flies at 4 hr following TBI. Only a few genes changed expression more than twofold in +/Rel del flies relative to +/+ and Rel del /Rel del flies, and they were not canonical innate immune response genes. Therefore, Rel is necessary for TBI-induced secondary injuries but in complex ways involving Rel gene dose, genetic background, diet, and possibly small changes in expression of innate immune response genes. Copyright © 2020 by the Genetics Society of America.