In COPD, body composition studies have focused primarily on low BMI. We examined obesity (BMI ≥ 30 kg/m(2)) as a risk factor for poor function and longitudinal functional decline. Data from a longitudinal cohort of adults with COPD (n = 1096) and an age- and sex-matched comparison group collected in two in-person visits ∼49 months apart were analyzed. Two measures of functioning were examined: six-minute walk distance (6MWD) and Short Physical Performance Battery (SPPB). Multivariate regression analyses examined relationships of obesity with functioning. Secondary analyses stratified by GOLD classification (GOLD-0/1, GOLD-2, GOLD-3/4). Obesity (53% of COPD cohort) was associated cross-sectionally with 6MWD and SPPB in COPD, and only with 6MWD in the comparison group. Obesity predicted significant functional decline in 6MWD for individuals with COPD (odds ratio (OR) for decline [95% CI] 1.8 [1.1, 2.9]), but not the comparison group. Secondary analyses revealed that the risk of decline was significant only in those with more severe COPD (GOLD 3/4, OR = 2.3 [1.0, 5.4]). Obesity was highly prevalent and was associated with poor function concurrently and with subsequent decline in 6MWD in COPD. Obesity in COPD should be considered a risk not only for more co-morbidities and greater health care use, but also for functional decline.