Fatigue is a common symptom in cancer patients receiving radiotherapy. However, previous studies report inconsistent patterns of fatigue change. The aim of this study was to estimate changes in fatigue among patients with cancer before, during, and after radiotherapy. Five databases (PubMed, SDOL, CINAHL Plus with Full Text, Medline [ProQuest], and ProQuest Dissertations) were searched for studies published from January 2006 to May 2021. Three effect sizes of fatigue change (immediate, short-term, and long-term) were calculated for each primary study using standardized mean difference. A random-effect model was used to combine effect sizes across studies. Subgroup analyses and meta-regression were performed to identify potential categorical and continuous moderators, respectively. Sixty-five studies were included in this meta-analysis. The weighted mean effect size for immediate, short-term, and long-term effects was 0.409 (p < .001; 95% CI [0.280, 0.537]), 0.303 (p < .001; 95% CI [0.189, 0.417]), and 0.201 (p = .05; 95% CI [-0.001, 0.404]), respectively. Studies with prostate cancer patients had a significantly higher short-term (0.588) and long-term weight mean effect size (0.531) than studies with breast (0.128, -0.072) or other cancers (0.287, 0.215). Higher radiotherapy dosage was significantly associated with a higher effect size for both immediate (β = .0002, p < .05) and short-term (β = .0002, p < .05) effect. Findings from this meta-analysis indicated that radiotherapy-induced fatigue (RIF) exist for more than 3 months after the completion of treatment. Assessment of radiation-induced fatigue in cancer patients should extend long after treatment completion, especially for patients with prostate cancer and patients receiving a higher radiation dose. Interventions to reduce fatigue tailored for different treatment phases may be developed.