Objective To estimate food insecurity (FI) prevalence among UK adults and investigate associations with socio-demographic characteristics, diet and health. Design Weighted cross-sectional survey data. FI was measured using the USDA Adult Food Security Survey Module. Data were analysed using adjusted logistic regression models. Setting United Kingdom. Participants 2551 participants (aged 18-64 years); sub-sample (n 1949) used to investigate association between FI and overweight. Results FI prevalence was 24·3 %. Higher odds of FI were observed among participants who reported that making ends meet was difficult v. easy (OR 19·76, 95 % CI 13·78, 28·34), were full-time students v. non-students (OR 3·23, 95 % CI 2·01, 5·18), had low v. high education (OR 2·30, 95 % CI 1·66, 3·17), were male v. female (OR 1·36, 95 % CI 1·01, 1·83) and reported their ethnicity as mixed (OR 2·32, 95 % CI 1·02, 5·27) and white other (OR 2·04, 95 % CI 1·04, 3·99) v. white British. Odds of FI were higher in participants living with children v. alone, especially in single-parent households (OR 2·10, 95 % CI 1·19, 3·70). Odds of FI decreased per year of increase in age (OR 0·95, 95 % CI 0·94, 0·96) and were lower in participants not looking for work v. full-time employed (OR 0·60, 95 % CI 0·42, 0·87). Food insecure v. food secure adults had lower odds of consuming fruits (OR 0·59, 95 % CI 0·47, 0·74) and vegetables (OR 0·68, 95 % CI 0·54, 0·86) above the median frequency, and higher odds for fruit juice (OR 1·39, 95 % CI 1·10, 1·75). Food insecure v. food secure adults had higher odds of reporting unhealthy diets (OR 1·65, 95 % CI 1·31, 2·10), poor general health, (OR 1·90, 95 % CI 1·50, 2·41), poor mental health (OR 2·10, 95 % CI 1·64, 2·69), high stress (OR 3·15, 95 % CI 2·42, 4·11) and overweight (OR 1·32, 95 % CI 1·00, 1·75). Conclusions FI prevalence was high and varied by socio-demographic characteristics. FI was associated with poorer diet and health.