Objective Polypharmacy is frequent among older cancer patients and increases the risk of potential drug-related problems (DRPs). DRPs are associated with adverse drug events, drug-drug interactions and hospitalisations. Since no standardised polypharmacy assessment methods for oncology patients exist, we aimed to develop one that can be integrated into routine care. Methods Based on the Systematic Tool to Reduce Inappropriate Prescribing (STRIP), we developed OncoSTRIP, which includes a polypharmacy anamnesis, a concise geriatric assessment, a polypharmacy analysis taking life expectancy into account and an optimised treatment plan. Patients >= 65 years with >= 5 chronic drugs visiting our outpatient oncology clinic were eligible for the polypharmacy assessment. Results OncoSTRIP was integrated into routine care of our older cancer patients. In 47 of 60 patients (78%), potential DRPs (n = 101) were found. In total, 85 optimisations were recommended, with an acceptance rate of 41%. It was possible to reduce the number of potential DRPs by 41% and the number of patients with at least one potential DRP by 30%. Mean time spent per patient was 71 min. Conclusions Polypharmacy assessment of older cancer patients identifies many pharmacotherapeutic optimisations. With OncoSTRIP, polypharmacy assessments can be integrated into routine care.