Objectives Sleep disturbances and fatigue are common in prostate cancer patients undergoing radiotherapy. Prior research suggests mind‐body techniques may improve these outcomes. We conducted a randomized controlled trial of qigong/tai chi (QGTC) in men with prostate cancer undergoing radiotherapy. Methods Men with prostate cancer starting definitive radiation were randomized to 1 of 3 groups: (1) QGTC; (2) light exercise (LE); or (3) waiting list control. Sleep disturbances (Pittsburgh Sleep Quality Index) and fatigue (Brief Fatigue Inventory) were assessed at baseline, midway through radiotherapy (T2), during the last week of radiotherapy (T3), and at 1 (T4) and 3 months (T5) after the end of radiotherapy. Patients in the QGTC and LE groups attended three 40‐minute classes per week throughout radiotherapy. Results Ninety patients were randomized to the 3 groups (QGTC = 26; LE = 26; waiting list control = 24). The QGTC group reported longer sleep duration midway through radiotherapy (QGTC = 7.01 h; LE = 6.42; WL = 6.50; P = .05), but this difference did not persist over time. There were no group differences in other domains of sleep or fatigue. Exploratory analyses conducted to examine the effect of health‐related quality of life (Expanded Prostate Cancer Index Composite and American Urological Association Symptom score) on sleep and fatigue showed significant correlations across multiple domains. Conclusions Qigong/tai chi during radiation for prostate cancer resulted in superior sleep duration midway through radiation, but this effect was not durable, and there were no differences in other domains of sleep or fatigue. Exploratory analysis demonstrated that both sleep and fatigue were highly correlated with prostate cancer–related physical symptoms. Future mind‐body intervention studies should incorporate multimodal therapy focused on improving physical symptoms in this population.