Purpose A comprehensive geriatric assessment (CGA) is a time-consuming approach that requires a special team and a screening test, whereas the G8 screening test is a practical and validated test for screening cancer patients. This study aimed to evaluate the validity and reliability of the G8 test in older patients without cancer and to investigate its concordance with CGA in an outpatient clinic. Methods Two hundred older patients were included in the study. CGA and G8 tests were performed, and the concordance between them was evaluated for scale validity using Spearman correlation coefficients (r) and kappa analyses. Patients who obtained scores lower than the predefined cutoff values in at least one of the CGA tests were considered to have an abnormal CGA. Inter-rater and intra-rater concordance were assessed for reliability. Results Of the 200 patients, 57.4% were female, and the median age was 73 (63-93) years. There was a strong concordance between the CGA and G8 screening test (kappa: 0.630;p < 0.001). Inter-rater and intra-rater concordance in the reliability assessments were high (kappa: 0.886; kappa: 875;p < 0.001, respectively), and inter- and intra-clinician assessments of the G8 scores revealed significant correlations (r = 0.962 andr = 0.976, respectively;p < 0.001). Conclusion The G8 screening test is a valid and reliable tool for older adults without malignancy. It is a quick and practical test for physicians who frequently admit older patients.