Syrian refugees in Lebanon are facing vulnerabilities that are affecting their food insecurity (FI) levels. The objectives of this study were to measure dietary diversity, FI and mental health status of Syrian refugee mothers in Lebanon and to explore its associations with their anaemia and nutritional status. A cross-sectional study was conducted among mothers with children under 5 years ( n 433) in Greater Beirut, Lebanon. Dietary diversity was measured using the Minimum Dietary Diversity for Women (MDD-W) of reproductive age and FI using the global Food Insecurity Experience Scale at the individual level. Depression and post-traumatic stress disorder (PTSD) were measured to assess the maternal mental health status. Data on socio-economic characteristics, anthropometric measurements and Hb concentrations were collected. Overall, 63·3 % of the mothers had a low dietary diversity (LDD) and 34·4 % were moderately to severely food insecure, with 12·5 % being severely food insecure. The prevalence of PTSD, moderate depression and severe depression was 13·2, 11·1 and 9·9 %, respectively. A significant correlation was found between LDD and FI ( P < 0·001). Low income was significantly associated with LDD and FI. Poor mental health was significantly associated with FI. LDD and FI were not associated with anaemia and nutritional status of mothers. Low-income households had significantly higher intakes of grains and refined starchy staples, whereas high-income households consumed more nutritious foods and sweets. Evidence of inadequate diet quality, FI and poor mental health among Syrian refugee mothers in Lebanon is presented. Multifaceted actions are needed to reduce FI and improve dietary diversity.