Background: US research organizations increasingly are supporting patient and stakeholder engagement in health research with a goal of producing more useful, relevant and patient-centered evidence better aligned with real-world clinical needs. The Patient-Centered Outcomes Research Institute (PCORI) engages patients, family caregivers and other health-care stakeholders, including clinicians, payers and policymakers, as active partners in prioritizing, designing, conducting and disseminating research as a key strategy to produce useful evidence for health-care decision making. Objective: To inform effective engagement practices and policies, we sought to understand what motivates patients and caregivers to engage as partners on PCORI-funded research projects and how such engagement changed their lives. Methods: We conducted thematic analysis of open-ended survey responses from 255 patients, family caregivers and individuals from advocacy and community-based organizations who engaged as partners on 139 PCORI-funded research projects focusing on a range of health conditions. Results: Partners' motivations for engaging in research were oriented primarily towards benefiting others, including a desire to improve patients' lives and to support effective health-care interventions. In addition to feeling they made a positive difference, many partners reported direct benefits from engagement, such as new relationships and improved health habits. Discussion and Conclusions: By identifying patient and caregiver motivations for engaging in research partnerships and what they get out of the experience, our study may help research teams and organizations attract partners and foster more satisfying and sustainable partnerships. Our findings also add to evidence that engagement benefits the people involved as partners, strengthening the case for more widespread engagement.