Objective <p>To examine the linear and non-linear associations between social capital and obesity in older adults after relocation for poverty alleviation in China.</p> Methods <p>Overall, 731 participants [mean age 71.58 (SD:6.81) years; 48.7% female] were included in this longitudinal study. Central obesity was assessed using the Body Roundness Index (BRI). The group-based trajectory model (GBTM) was used to identify trajectories of social capital and its components. Logistic regression models were used to assess the associations between social capital and BRI. We also used restricted cubic spline (RCS) to examine the non-linear associations between social capital and BRI.</p> Results <p>Three structural social capital (SSC) trajectories (low, moderate, and high), two relational (RSC) and cognitive social capital (CSC) (low and high) trajectories were identified by GBTM. In joint analyses, the high-stable RSC trajectory group was associated with a significantly higher BRI compared with the low-stable group (<i>β</i> = 0.379, <i>95% CI</i>: 0.026, 0.731). But no significant associations were found between SSC/CSC and BRI. In addition, RCS analysis revealed a U-shaped relationship between RSC (scale: 0–12 score) and BRI, with an inflection point at score of 6.211 [mean RSC 9.317 (SD: 3.578) score], which falls at the lower end of the RSC distribution. This U-shaped association was also observed across subgroups stratified by sex, age, and monthly income levels, although the specific inflection points varied.</p> Conclusions <p>In this population-based longitudinal study of relocated older adults, a U-shaped relationship was observed between RSC and BRI. These observed relationships highlight the need for primary medical institutions to consider RSC when assessing obesity in relocated older adults.</p>