Objectives: Nearly 1 in 3 cancer patients undergoing radiotherapy experience anxiety, which can negatively affect both quality of life and prognosis. This systematic review aimed to synthesize evidence on patient education and its impact on anxiety levels among cancer patients undergoing radiotherapy. Methods: We conducted a systematic review of the current literature using 3 scientific databases—PubMed, EBSCOhost, and Web of Science—to identify intervention studies. Data were analyzed using a narrative synthesis approach, integrating findings across studies to draw comprehensive conclusions. Results: Fifteen studies involving 1988 participants were included. Baseline anxiety levels, measured using various instruments, ranged from 2.9% to 88.2% before radiotherapy and declined thereafter in both intervention and control groups. Most studies reported no significant difference in anxiety reduction between intervention groups receiving educational programs and control groups receiving standard education. However, interventions that were more intensive and delivered by professional staff were more likely to produce statistically significant reductions in anxiety. Conclusions This review suggests that patient education remains relevant for reducing anxiety among cancer patients undergoing radiotherapy. Routine screening for anxiety prior to radiotherapy is recommended, along with the development of more intensive and tailored educational interventions rather than passive or minimal approaches.