Neighbourhood social, economic and environmental factors are associated with childhood obesity. Childhood obesity disproportionately impacts those living in low-income neighbourhoods. Childhood obesity prevalence is strongly associated with community-level social and economic conditions as measured using a composite Economic Hardship Index. Childhood obesity prevalence among communities in the highest hardship quartile was more than double the prevalence among communities in the lowest hardship quartile (26.9 vs. 12.5%). The relationship between economic hardship and obesity differs by race/ethnicity. The association between community-level economic hardship and childhood obesity prevalence was examined in Los Angeles County, one of the largest and most racially and ethnically diverse regions in the United States. Data from the 2008-2009 California Department of Education's Physical Fitness Testing Program were analyzed to assess obesity prevalence among 5th, 7th and 9th grade students attending public schools (n = 298,485). Community-level socioeconomic conditions were compared using a census-tract-based Economic Hardship (EH) index. Mixed-effects modelling was used to examine the association between obesity prevalence and gender, grade, race/ethnicity and EH. Higher community-level EH was associated with higher childhood obesity prevalence (P < 0.001). The obesity prevalence among communities in the highest EH quartile (26.9%) was more than double the obesity prevalence among communities in the lowest EH quartile (12.5%). The slope of the association between EH and childhood obesity differed by racial/ethnic group. The slope was higher for non-Hispanic White students, Hispanics, and non-Hispanic Asians, and lower for non-Hispanic Black students. Racial/ethnic disparities were observed across the socioeconomic spectrum. Findings suggest that efforts to improve community socioeconomic conditions could reduce childhood obesity prevalence. Prevention efforts should target communities with high economic hardship and also focus on providing culturally relevant interventions that address disparities in obesity prevalence across communities.