OBJECTIVES: We aimed to assess the level of prior SARS-CoV-2 infection in socially deprived neighbourhoods after the first wave of the pandemic, and to identify factors associated with seropositivity. DESIGN: A cross-sectional study. SETTING: Three socially deprived neighbourhoods of the city of Perpignan, in the south of France, where large settled Roma communities live. PARTICIPANTS: People aged 6 years old or over, living in the study area. 700 people were included in the study using two-stage stratified sampling design. INTERVENTIONS: The study included a questionnaire and SARS-CoV-2 antibody testing by the Roche Elecsys immunoassay between 29 June and 17 July 2020. PRIMARY AND SECONDARY OUTCOME MEASURES: SARS-CoV-2 antibody seroprevalence was estimated from weighted data. Associated factors and reported symptoms were investigated using univariable and multivariable logistic regressions. RESULTS: The seroprevalence of anti-SARS-CoV-2 antibodies was 35.4% (95% CI 30.2% to 41.0%). People aged 15-64 years old had increased odds of being seropositive than those aged 65 years or over. Obese people had higher odds of being seropositive (adjusted OR (aOR)=2.0, 95%?CI 1.1 to 3.8). The odds of being seropositive were higher in households with clinical COVID-19 cases (one case: aOR=2.5, 95%?CI 1.3 to 5.0; several cases: aOR=6.9, 95%?CI 3.1 to 15.2). In the neighbourhood with the highest measured seroprevalence, people living in a dwelling with one to two rooms had higher odds of being seropositive than those living in a four-room house (aOR=2.8, 95%?CI 1.2 to 6.3). Working during the lockdown was associated with lower odds of being seropositive (aOR=0.2, 95%?CI 0.03 to 1.0). CONCLUSION: Transmission of SARS-COV-2 in this vulnerable population was very high during the COVID-19 pandemic's first wave. Our results highlight the need to strengthen and adapt preventive measures taking into account all social determinants of health, especially housing conditions.