Purpose <p>Nearly 30% of prostate cancer patients undergo external beam radiotherapy (EBRT) as first-line therapy; however, 27–53% of localized cases experience recurrence or progression. Circadian rhythm regulates 24-h biological cycles, including cell growth, DNA repair, metabolism, and immune function. Clinical evidence suggests that radiotherapy efficacy and toxicity may vary by time of day.</p> Methods <p>This retrospective cohort study evaluated whether EBRT time of day affects treatment outcomes and toxicity in localized prostate cancer. A total of 224 patients treated between 2012 and 2021 at the Portuguese Oncology Institute of Porto were analyzed. Patients were divided into two groups based on the receiver operating characteristic (ROC) curve for metastasis-free survival using treatment time (before or after 12:45PM), and in a secondary analysis, morning (08:30–11:30AM) versus afternoon (03:00–08:00PM). Survival outcomes and toxicities were assessed using Kaplan-Meier curves, Cox proportional hazards models, and binary logistic regression. Propensity score matching was used to adjust for confounding variables.</p> Results <p>Morning treatment was significantly associated with a 76% reduced risk of metastasis post-treatment (<i>P</i> = 0.031) and improved 10-year metastasis-free survival (log-rank <i>P</i> = 0.014). Furthermore, patients treated in the morning had significantly lower risks of late grade ≥ 2 gastrointestinal (86% reduction, <i>P</i> = 0.013) and genitourinary (67% reduction, <i>P</i> = 0.003) toxicity.</p> Conclusion <p>Despite the retrospective, single-institution design, our results indicate that EBRT timing significantly influences prostate cancer outcomes.</p>