Background: Prostate cancer remains a leading cause of cancer-related deaths in men, with radioresistance limiting treatment efficacy. This study investigates the role of Calbindin 1 (CALB1), a calcium-binding protein regulated by miR-186-5p, in prostate cancer progression and radiation response. Methods: CALB1 expression was analyzed using GEO and TCGA datasets, and the regulatory relationship with miR-186-5p was validated. Functional studies including CALB1 knockdown, calcium chelation, and mitochondrial rescue interventions were conducted in prostate cancer cells, spheroids, and xenograft models, assessing proliferation, senescence, calcium homeostasis, and radiation response. Results: We identified CALB1 as a target of downregulated miR-186-5p in prostate cancer. CALB1 silencing inhibited prostate cancer growth by inducing cellular senescence through calcium dysregulation, mitochondrial dysfunction, and oxidative stress. CALB1 depletion significantly enhanced radiosensitivity both in vitro and in vivo, with calcium chelation or mitochondrial interventions partially rescuing these effects. Conclusions: CALB1 regulates prostate cancer progression and radiation response by maintaining calcium homeostasis. Its depletion triggers calcium overload and mitochondrial dysfunction, enhancing radiation sensitivity and identifying CALB1 as a potential therapeutic target.