The ability to remember future actions (prospective memory) is an important determinant of daily functioning in older adults. While social wellbeing is associated with better cognitive function generally, it is unknown how social wellbeing affects, and is affected by, prospective memory. Using a two-wave longitudinal design, we investigated the relationship between prospective memory and social wellbeing over 3 years. Data come from the first two waves of the Canadian Longitudinal Study on Aging (n = 23,609, age: 45-86), collected between 2011-2018. Event-based and time-based prospective memory were measured using a standardized laboratory task, and social wellbeing was operationalized as self-reported social support and social participation. We used latent change score modeling to evaluate longitudinal associations between prospective memory and social wellbeing. There was a positive bidirectional relationship with higher baseline social support predicting better prospective memory, and vice-versa. However, no similar relationship was observed for social participation. Time-based and event-based prospective memory did not differ in their association with social wellbeing. Social support may buffer stress and promote a richer, mentally stimulating environment, thereby improving prospective memory. Conversely, better prospective memory may increase social support, for example, by reducing the risk of memory lapses interfering with supportive relationships (e.g., forgetting appointments).