Abstract Background The EQ-5D-3L and EQ-5D-5L are two generic health-related quality of life measures, which may be used in clinical and health economic research. They measure impairment in 5 aspects of health: mobility, self-care, usual activities, pain/discomfort and anxiety/depression. The aim of this study was to assess the performance of the EQ-5D-3L and EQ-5D-5L in measuring the self-reported health status of older patients with substantial multimorbidity and associated polypharmacy. Methods Between 2017 and 2019, we administered EQ-5D-3L and EQ-5D-5L to a subset of patients participating in the OPERAM trial at 6 months and 12 months after enrolment. The OPERAM trial is a two-arm multinational cluster randomised controlled trial of structured medication review assisted by a software-based decision support system versus usual pharmaceutical care, for older people (aged ≥ 70 years) with multimorbidity and polypharmacy. In the psychometric analyses, we only included participants who completed the measures in full at 6 and 12 months. We assessed whether responses to the measures were consistent by assessing the proportion of EQ-5D-5L responses, which were 2 or more levels away from that person’s EQ-5D-3L response. We also compared the measures in terms of informativity, and discriminant validity and responsiveness relative to the Barthel Index, which measures independence in activities of daily living. Results 224 patients (mean age of 77 years; 56% male) were included in the psychometric analyses. Ceiling effects reported with the EQ-5D-5L (22%) were lower than with the EQ-5D-3L (29%). For the mobility item, the EQ-5D-5L demonstrated better informativity (Shannon’s evenness index score of 0.86) than the EQ-5D-3L (Shannon’s evenness index score of 0.69). Both the 3L and 5L versions of EQ-5D demonstrated good performance in terms of discriminant validity, i.e. (out of all items of the EQ-5D-3L and EQ-5D-5L, the pain/discomfort and anxiety/depression items had the weakest correlation with the Barthel Index. Both the 3L and 5L versions of EQ-5D demonstrated good responsiveness to changes in the Barthel Index. Conclusion Both EQ-5D-3L and EQ-5D-5L demonstrated validity and responsiveness when administered to older adults with substantial multimorbidity and polypharmacy who were able to complete the measures.