BackgroundBoth obesity and depression pose significant public health challenges. Understanding the age-gender differences in the BMI-depression associations is crucial, especially in the Saudi context, where cultural norms, lifestyle factors, and socio-economic conditions may significantly influence these associations. We aimed to explore and elucidate these associations in Saudi adults, considering age and gender as crucial factors.MethodsThe research utilized a cross-sectional web-based survey to examine the impact of age and gender on the associations between BMI and depression. The study included 4,683 Saudi adults aged 18–60 years. The questionnaire assessed socio-demographics, lifestyle factors, chronic illnesses, and self-reported height and weight to calculate BMI. Depression Inventory-II (BDI-II), a self-reported scale, was used to evaluate depression, categorizing participants into depression and non-depression groups. A snowball convenience sampling was utilized, with clear inclusion and exclusion criteria established to mitigate potential biases. Data analysis was stratified by age and gender, and binary logistic regression was utilized to determine the associations between depression and BMI, incorporating odds ratios (ORs) in both crude and adjusted forms, along with 95% confidence intervals (CIs).ResultsThe prevalence of depression was found to be 43.3%, while 21% of participants were classified as obese. Notably, individuals in the depression group were significantly older, predominantly female, and more likely to reside in the Western and Southern regions of Saudi Arabia. They had lower family income, were more frequently non-smokers, had a higher prevalence of chronic diseases, and had higher BMI. Moreover, obese individuals had significantly higher depression scores (p < 0.05). After adjusting for confounding factors, OR for depression was significantly higher among obese individuals (1.92 [1.56–3.36]). Stratified analysis by age and gender revealed higher ORs for depression among obese participants in older age and among women.ConclusionThis study highlights a significant association between obesity and increased prevalence of depression, with notable age and gender differences in this association. The findings suggest that older adults and women, particularly those with obesity, are at a higher risk for depression. These insights emphasize the importance of tailoring public health strategies and mental health interventions to address the specific needs of these vulnerable groups in Saudi Arabia.