Reducing socioeconomic inequalities in obesity is a public health priority. We examined whether population density, a key environmental characteristic associated with obesity, moderates the associations of area-level socioeconomic status (SES) with waist circumference and its rate of change. Data from adults in an Australian national study, collected across three waves (1999-2000, 2005-06, 2011-12), were used. Outcomes were objectively measured baseline waist circumference (<i>N</i> = 4835) and its rate of change (<i>N</i> = 1158). Exposure was baseline area-level SES. Baseline population density was calculated within a 1 km buffer around residences. Multilevel regression models were used. Higher area-level SES was associated with smaller baseline waist circumference (-1.45, 95 % CI [-2.28, -0.12]) but not with change over time (0.04, 95 % CI [-0.02, 0.09]). In stratified cross-sectional analyses, this association was significant in lower- (-5.00, 95 % CI -7.73, -2.28]) and medium-density (-2.61, 95 % CI [-4.55, -0.67]), but not in higher-density areas (-0.54, 95 % CI [-1.73,0.65]). In longitudinal analyses, lower SES was linked to smaller increases in waist circumference in lower- (0.16, 95 % CI [0.03, 0.29]) and medium-density (0.17, 95 % CI [0.05, 0.30]) areas, suggesting a narrowing of socioeconomic inequalities in obesity. However, annual changes were modest and unlikely to offset baseline inequalities. Population density moderated associations of area-level SES with waist circumference and its change. Inequalities in baseline obesity were pronounced in lower density areas but not higher density areas. While the longitudinal findings of the moderation patterns were different from baseline results, they suggest persistent socioeconomic inequalities in obesity in lower density areas.