Elucidating the mechanisms involved in sensitizing radioresistant tumors to ionizing radiation (IR) treatments while minimizing injury to surrounding normal tissue is an important clinical goal. Due to their sequence-derived specificity and properties as gene regulators in IR-affected pathways, microRNAs (miRNAs) could serve as adjuvant therapeutic agents that alter cellular sensitivity to radiation treatment. To identify radiosensitizing miRNAs, we initially utilized the Caenorhabditis elegans vulval cell model, an in vivo system developed to study IR-dependent radiosensitivity as a measure of clonogenic cell death. We tested several candidate miRNA-deletion mutants post γ-irradiation and identified cel-mir-237 as a miRNA which when deleted caused animals to be more resistant to IR, whereas cel-mir-237 overexpressing strains were IR sensitive. In addition, wild-type animals downregulated cel-mir-237 levels post IR in a time-dependent manner. We identified jun-1 (JUN transcription factor homolog) as a novel target of cel-mir-237. Specifically, jun-1 transcript levels increased in wild-type animals post γ-irradiation, and loss of cel-mir-237 also resulted in higher jun-1 expression. As expected, loss of jun-1 resulted in IR sensitivity, similar to the phenotype of cel-mir-237 overexpressors. As miR-237 is the homolog of human miR-125, we validated our findings in MCF-7 and MDA-MB-231 breast cancer cell lines, which harbor lower hsa-miR-125b levels than normal human mammary epithelial cells (HMECs). Forced expression of hsa-miR-125b in these cells resulted in radiosensitivity, as seen by reduced clonogenic survival, enhanced apoptotic activity and enhanced senescence post IR. Finally, re-expression of c-JUN in MDA-MB-231 cells promoted radioresistance and abrogated miR-125-mediated radiosensitization. Our findings suggest that overexpression of cel-mir-237 and its homolog, hsa-miR-125b, functions as sensitizers to γ-irradiation in both a nematode in vivo model and breast cancer cells, and could potentially be utilized as an adjuvant therapeutic to enhance radiation sensitivity.