Background <p>Population aging has raised growing concerns about the psychological well-being of middle-aged and older adults. Although loneliness and social participation are closely linked, their long-term interplay remains insufficiently understood. This study examined the dynamic associations between loneliness and social participation among Chinese middle-aged and older adults.</p> Methods <p>Data were drawn from 6,758 participants who participated in five waves of the China Health and Retirement Longitudinal Survey (CHARLS, 2011–2020). Loneliness and social participation were assessed at each wave. Both variable-centered and person-centered approaches were used to examine reciprocal associations, developmental trends, and heterogeneity in joint trajectories of loneliness and social participation.</p> Results <p>Loneliness and social participation were reciprocally associated over time. Moreover, the intercept of social participation was negatively associated with changes in loneliness, whereas the loneliness intercept was not associated with changes in social participation. Furthermore, five distinct joint trajectories of loneliness and social participation were identified.</p> Conclusion <p>Sustained social participation may help protect middle-aged and older adults from loneliness. Individuals with persistently low or declining levels of social participation appear to be at greater risk and may require additional support. Importantly, the identified trajectory patterns underscore the need for subgroup-specific strategies, including maintaining meaningful relationships for low-risk individuals and implementing proactive outreach and engagement strategies for those at risk of increasing loneliness.</p>