Abstract Background Polypharmacy is an important issue in the care of older patients with cancer, as it increases the risk of unfavorable outcomes. We estimated the prevalence of polypharmacy, potentially inappropriate medication (PIM) use, and drug–drug interactions (DDIs) in older patients with cancer in Korea and their associations with clinical outcomes. Subjects, Materials, and Methods This was a secondary analysis of a prospective observational study of geriatric patients with cancer undergoing first-line palliative chemotherapy. Eligible patients were older adults (≥70 years) with histologically diagnosed solid cancer who were candidates for first-line palliative chemotherapy. All patients enrolled in this study received a geriatric assessment (GA) at baseline. We reviewed the daily medications taken by patients at the time of GA before starting chemotherapy. PIMs were assessed according to the 2015 Beers criteria, and DDIs were assessed by a clinical pharmacist using Lexi-comp Drug Interactions. We evaluated the association between polypharmacy and clinical outcomes including treatment-related toxicity, and hospitalization using logistic regression and Cox regression analyses. Results In total, 301 patients (median age 75 years; range, 70–93) were enrolled; the most common cancer types were colorectal cancer (28.9%) and lung cancer (24.6%). Mean number of daily medications was 4.7 (±3.1; range, 0–14). The prevalence of polypharmacy (≥5 medications) was 45.2% and that of excessive polypharmacy (≥10 medications) was 8.6%. PIM use was detected in 137 (45.5%) patients. Clinically significant DDIs were detected in 92 (30.6%) patients. Polypharmacy was significantly associated with hospitalization or emergency room (ER) visits (odds ratio: 1.73 [1.18–2.55], p < .01). Neither polypharmacy nor PIM use showed association with treatment-related toxicity. Conclusion Polypharmacy, PIM use, and potential major DDIs were prevalent in Korean geriatric patients with cancer. Polypharmacy was associated with a higher risk of hospitalization or ER visits during the chemotherapy period.