Objectives. We investigated relations between changes in neighborhood ethnic composition and changes in body mass index (BMI) and waist circumference among Chinese and Hispanic immigrants in the United States. Methods. We used Multi-Ethnic Study of Atherosclerosis data over a median 9-year follow-up (2000-2002 to 2010-2012) among Chinese (n = 642) and Hispanic (n = 784) immigrants aged 45 to 84 years at baseline. We incorporated information about residential moves and used econometric fixed-effects models to control for confounding by time-invariant characteristics. We characterized neighborhood racial/ethnic composition with census tract-level percentage Asian for Chinese participants and percentage Hispanic for Hispanic participants (neighborhood coethnic concentration). Results. In covariate-adjusted longitudinal fixed-effects models, results suggested associations between decreasing neighborhood coethnic concentration and increasing weight, although results were imprecise: within-person BMI increases associated with an interquartile range decrease in coethnic concentration were 0.15 kilograms per meters squared (95% confidence interval [CI] = 0.00, 0.30) among Chinese and 0.17 kilograms per meters squared (95% CI = -0.17, 0.51) among Hispanic participants. Results did not differ between those who did and did not move during follow-up. Conclusions. Residential neighborhoods may help shape chronic disease risk among immigrants.