Due to the growing number of people aging with multiple sclerosis (MS), there has been a call for rehabilitation specially targeted older adults with MS in order to support them in better wellbeing, despite physical and cognitive impairment. However, the existing research within the area of rehabilitation has primarily focused on the physical and psychological aspects of aging with MS, omitting the social element. This study aims to examine how social relations and engagement in leisure activities predict wellbeing among older adults with MS living in Denmark. Furthermore, the study aims to identify which sociodemographic and health-related factors are the most important in predicting whether older adults with MS face challenges in participating in leisure activities and experiencing different kinds of social relations. A cross-sectional survey was designed to measure social relations, wellbeing, and engagement in leisure activities among older adults with MS. Of the 4,329 people over 65 years diagnosed with MS in Denmark in 2022, 2,574 (59.46%) were invited to participated in the study, and 1,107 (43.03%) ended up answering the survey. Linear and logistic regression analyses and dominance analyses were conducted to examine the associations between wellbeing, leisure activities, social relations, sociodemographic and health-related factors. The results of the study show that perceived emotional social support (mean difference 8.69, 95% CI 5.23; 12.14) and perceived instrumental social support (mean difference 4.15, 95% CI 0.95; 7.35), were associated with better wellbeing among older adults with MS. Perceived strained social relations (mean difference -7.95, 95% CI -10.66; -5.26) were on the contrary associated with lower levels of wellbeing. Strained social relations were the most important predictors of wellbeing accounting for 59% of the predicted variance. Experiencing social emotional support from friends, coworkers, or neighbors (39% of the predicted variance), experiencing instrumental social support from children or children in law (43% of the predicted variance), and experiencing strained social relations with partner (48% of the predicted variance) constituted he most important predictor of wellbeing. Engagement in five out of fourteen leisure activities were associated with better wellbeing among the participants. The leisure activities there was found to be the most important predictor of wellbeing represented both social (37% of the predicted variance), physical (18% of the predicted variance), and creative elements (13% of the predicted variance). Finally, cohabitation was found to be the most important predictor of having perceived emotional social support (59% of the predicted variance), instrumental social support (78.9% of the predicted variance) and strained social relations (18.8% of the predicted variance) and mobility was found to be the most important predictor of challenges in participating in leisure activities (81.8% of the predicted variance). The findings of the study highlight that rehabilitation targeting older adults with MS should focus on both physical, psychological, and social elements of peoples' everyday life. Further, the results indicate that future rehabilitation focusing on social elements of aging with MS should take into account health and sociodemographic characteristics such as cohabitation, mobility, age, and sex, as these potentially relate to participation in leisure activities as well as social relations among older adults.