Polypharmacy is particularly prevalent in the elderly. The interest in this issue is growing, and many interventions exist to improve the appropriate use of polypharmacy for older people. However, evidence of their effectiveness is still limited. Thus, the aim of this study, based on a qualitative approach, was to identify the key elements perceived to influence the prescribing and dispensing of appropriate polypharmacy to older people in primary care. Semistructured interviews were conducted with general practitioners and community pharmacists practicing in the region of Nouvelle-Aquitaine (France). Pre-existing topic guides based on the 12 TDF domains have been adapted and used. Data were analyzed using the framework method and content analysis. A focus group of healthcare professionals was conducted, and behavior change techniques (BCTs) were used to select the intervention components. Seventeen interviews were convened. A wide range of determinants were identified as barriers and/or facilitators. Nine domains were selected as key domains to target for intervention. Five intervention components (behavior change techniques—BCTs) to include in an intervention were finally selected. The results of this study will serve as a starting point for the design of a theory-based intervention targeting healthcare professionals to improve appropriate prescribing and dispensing of polypharmacy for older people in primary care.