BACKGROUND: While multiple studies have evaluated the effect of body mass index (BMI) on the prevalence of gastrointestinal (GI) symptoms, data from Saudi Arabia are scarce. This study aimed to evaluate the association between GI symptoms and BMI in a Saudi population. METHODS: A prospective cross-sectional study was conducted between September 2019 and April 2020. The data were collected using an electronic self-administered questionnaire. The study included adult participants and collected data on patients' demographics and GI symptoms. Participants with underlying GI diseases were excluded. A multivariate regression analysis was used to report the adjusted prevalence of GI symptoms in different BMI categories. RESULTS: A total of 4415 participants completed the survey. After applying the exclusion criteria, 3866 were included. The mean age was 26.3 (±8.8) and 58.2% were females. The mean BMI was 25.2 (±6.7), and the distribution of BMI was as follows: underweight 428 (11.1%), normal BMI 1789 (46.2%), overweight 912 (23.5%), and obese 737 (19.1%). After adjustment for age, gender, and smoking and coffee habits, obesity (BMI > 30) was significantly associated with heartburn ( < 0.01, aOR 1.6, 95% CI: 1.33 - 1.92), bloating ( < 0.01, aOR 1.31, 95% CI: 1.08 - 1.6), and diarrhea ( < 0.01, aOR 1.72, 95% CI: 1.36 - 2.17)). Underweight (BMI < 18.5) was significantly associated with abdominal pain ( < 0.01, aOR 1.4, 95% CI: 1.12 - 1.73), nausea ( < 0.01, aOR 1.6, 95% CI: 1.29 - 2.1), and vomiting ( < 0.01, aOR 2.02, 95% CI: 1.23 - 3.25). There was no significant association between BMI and constipation. CONCLUSION: Obesity was associated with heartburn, diarrhea, and bloating, while underweight status was associated with nausea, vomiting, and abdominal pain. No association between BMI and constipation was found.