<p>This study aimed to investigate the developmental trajectories and reciprocal relationship between social activities and cognition in middle-aged and elderly people with chronic diseases after controlling for the differences among individuals. The study used a representative sample of adults older than 45 years from the China Health and Retirement Longitudinal Study (CHARLS). The parallel latent growth curve models (LGCMs) were used to investigate the developmental trajectories and interactions between social activities and cognitive function from a dynamic perspective. The random-intercept cross-lagged panel models (RI-CLPMs) were conducted to examine the longitudinal bidirectional relationship between social activities and cognitive function. There were 4,174 over the age of 45 chronic disease patients with the mean age of 57.10 ± 7.57 years. The parallel LGCMs demonstrated that the rate of change in social activities positively predicted the rate of change in cognitive function (<i>β</i> (95%CI) = 0.253 (0.132, 0.375)), and vice versa (<i>β</i> (95%CI) = 0.231 (0.121, 0.342)). At the within-person level, the cross-lagged analysis indicated temporal and bidirectional relationships between social activities and cognitive function (<i>β</i>_{<i>social activities→cognitive function</i>} = 0.045, <i>β</i>_{<i>cognitive function→social activities</i>} = 0.058, <i>P</i> &lt; 0.050) in 2015 and 2018. At the between-person level, a higher social activities level was associated with better cognitive function level during 10 years (<i>β</i> = 0.320, <i>P</i> &lt; 0.001). This study reveals that there is bidirectional temporal relationship between social activities and cognition among the middle-aged and elderly people with chronic diseases. Targeted interventions should be implemented to effectively slow cognitive decline and enhance their quality of life.</p>