Despite years of attention, avoiding medication-related harm remains a global challenge. Nursing homes provide essential health care for frail older individuals, who often experience multiple chronic diseases and polypharmacy, increasing their risk of medication errors. Evidence of effective interventions to improve medication safety in these settings is inconclusive. Focusing on patient safety culture is a potential key to intervention development as it forms the foundation for overall patient safety and is associated with medication errors. This study aims to develop an intervention to improve medication safety for nursing home residents through a cocreative process guided by integrated knowledge translation and experience-based codesign. This study used a cocreative process guided by integrated knowledge translation and experience-based co-design principles. Evidence on patient safety culture was used as an inspirational source for exploration of medication safety. Data collection involved semistructured focus groups to generate experiential knowledge (stage 1) to inform intervention design in a multidisciplinary workshop (stage 2). Research validation engaging different types of research expertise and municipal managerial representatives in finalizing the intervention design was essential. Acceptance of the final intervention for evaluation was aimed for through contextualization focused on partnership with a municipal advisory board. An abductive, rapid qualitative analytical approach to data analysis was chosen using elements from analyzing in the present, addressing the time-dependent, context-bound aspects of the cocreative process. Experiential knowledge was represented by three main themes: (1) closed systems and gaps between functions, (2) resource interpretation and untapped potential, and (3) community of medication safety and surveillance. The main themes informed the design of preliminary intervention components in a multidisciplinary workshop. An intervention design process focused on research validation in addition to contextualization resulted in the Safe Medication in Nursing Home Residents (SAME) intervention covering (1) campaign material visualizing key roles and responsibilities regarding medication for nursing home residents and (2) "Medication safety reflexive spaces" focused on social and health care assistants. The cocreative process successfully resulted in the multifaceted SAME intervention, grounded in lived experiences shared by some of the most important (but often underrepresented in research) stakeholders: frontline health care professionals and representatives of nursing home residents. This study brought attention toward closed systems related to functions in medication management and surveillance, not only informing the SAME intervention design but as opportunities for further exploration in future research. Evaluation of the intervention is an important next step. Overall, this study represents an important contribution to the complex field of medication safety. RR2-10.2196/43538.