BACKGROUND: The association between social capital and trajectories of cognitive function (CF) is still unclear among older adults in mainland China. The present study aims to examine the association using a longitudinal cohort from the Ningxia Healthy Aging Cohort. METHODS: Four waves of longitudinal data were abstracted to identify the CF trajectories using the conditional latent class growth model (LCGM). CF was measured using the Chinese version of the Mini-Mental State Examination (MMSE) from 2012 to 2023. Social capital, including social cohesion, social interaction, civic participation, and spiritual capital, was assessed using a series of standard instruments at baseline. Multinomial logistic regression was used to test the association between social capital and each trajectory class. RESULTS: Three distinct CF trajectories were identified: Stable (72.5%), slow decline (25.8%), and deterioration (1.7%). High spiritual capital was associated with a decreased risk of developing a deterioration trajectory (OR = 0.36, 95% CI = 0.14-0.93), while high civic participation (OR = 0.61, 95% CI = 0.46-0.81) was associated with a decreased risk of developing a slow decline trajectory. Low CF at baseline was the first important risk factor, followed by age, civic participation, sex, monthly income, spiritual capital, and obesity. CONCLUSIONS: The CF trajectories are heterogeneous among older Chinese adults. Civic participation was associated with a lower risk of a slow decline trajectory, and spiritual capital was slightly associated with a lower risk of a deterioration trajectory. These findings support the development of social-integrated interventions for vulnerable groups.