Background: Polypharmacy and potentially inappropriate medications (PIMs) are prevalent in older adults with cancer, but their associations with physical function are not often studied. This study examined the associations of polypharmacy and PIMs with physical function in older adults with cancer, and determined the optimal cutoff value for the number of medications most strongly associated with physical functional impairment. Methods: This cross-sectional analysis used baseline data from a randomized study enrolling patients aged ≥70 years with advanced cancer starting a new systemic cancer treatment. We categorized PIM using 2015 American Geriatrics Society Beers Criteria. Three validated physical function measures were used to assess patient-reported impairments: activities of daily living (ADL) scale, instrumental activities of daily living (IADL) scale, and the Older Americans Resources and Services Physical Health (OARS PH) survey. Optimal cutoff value for number of medications was determined by the Youden index. Separate multivariate logistic regressions were then performed to examine associations of polypharmacy and PIMs with physical function measures. Results: Among 439 patients (mean age, 76.9 years), the Youden index identified ≥8 medications as the optimal cutoff value for polypharmacy; 43% were taking ≥8 medications and 62% were taking ≥1 PIMs. On multivariate analysis, taking ≥8 medications was associated with impairment in ADL (adjusted odds ratio [aOR], 1.64; 95% CI, 1.01–2.58) and OARS PH (aOR, 1.73; 95% CI, 1.01–2.98). PIMs were associated with impairments in IADL (aOR, 1.72; 95% CI, 1.09–2.73) and OARS PH (aOR, 1.97; 95% CI, 1.15–3.37). A cutoff of 5 medications was not associated with any of the physical function measures. Conclusions: Physical function, an important component of outcomes for older adults with cancer, is cross-sectionally associated with polypharmacy (defined as ≥8 medications) and with PIMs. Future studies should evaluate the association of polypharmacy with functional outcomes in this population in a longitudinal fashion.