Background: Chemoradiotherapy (CRT) is the main treatment for elderly patients with non-metastatic rectal cancer who are ineligible for or decline surgery, but the optimal modality remains unclear. Objectives: This study was to validate the safety and efficacy of comprehensive geriatric assessment (CGA) guided radiotherapy in older patients. Design: An exploratory analysis of a single-arm, multicenter, Phase II trial. Methods: Patients aged over 70 and diagnosed with rectal cancer were enrolled and evaluated by CGA. CGA-guided radiotherapy was individually conducted in a multidisciplinary setting. Patients in fit, intermediate, and frail groups were scheduled to receive CRT, long-course radiotherapy, and short-course radiotherapy (SCRT) alone respectively. Patients who were unfit for or refused surgery were analyzed for acute toxicities and survival outcomes. Results: In a total of 109 enrolled patients, 47 individuals who did not undergo surgery were included, with 26, 9, and 12 categorized into fit, intermediate, and frail groups. Only 11 (23.4%) grade 3 or above toxicities were observed overall. Within a median follow-up of 69.0 months, the 3-year overall survival (OS), progression-free survival (PFS), and cancer-specific survival (CSS) rates were 44.3% (95% CI: 32.1%-61.2%), 25.5% (95% CI: 15.7%-41.6%) and 61.0% (95% CI: 47.8%-77.6%) in total. The 5-year OS, PFS, and CSS reached 15.0% (95% CI: 7.4%-30.3%), 14.6% (95% CI: 7.3%-29.4%), and 36.2% (95% CI: 22.0%-59.4%), with no significant difference among the three subgroups. SCRT (p < 0.001) and dose boost (p = 0.045) contributed to lower tumor-related death rates in multiple competing risk regressions. Conclusion: Radiotherapy guided by CGA was effective and well-tolerated in non-surgical elderly patients. SCRT alone seemed to achieve similar clinical outcomes as CRT in corresponding subgroups. However, given the limited size of this study, further investigation in a larger population is still needed for this strategy.