Objectives To evaluate the association between ultrasound assessment of gallbladder wall thickness (GBWT) among severe dengue patients and dengue patients with warning signs to their clinical outcomes. Methods A prospective, cross-sectional study involving adult dengue patients presented to our emergency department between March until September 2018. The patients were classified based on WHO classification. A gallbladder wall scan was performed on all patients. Results A total of 44 patients were enrolled into the study; majority of the patients with GBWT had severe dengue, significantly more than the dengue patients with warning signs (90.5% sensitivity; 69.6% specificity). The sensitivity of GBWT in determining admission to critical care areas or general ward was 100% with a specificity of 62.1%. Our analysis showed that the two variables significant in determining the severity of dengue were age (p = 0.045) and GBWT (p < 0.001). Both factors together gave 81.0% sensitivity and 78.3% specificity in predicting patients for severe dengue. The receiver operator characteristic curve revealed that using variable GBWT status can discriminate 87.1% (95%CI 66.3, 93.7%) of having severe dengue or dengue with warning signs. Conclusion The finding of GBWT when consolidated with other clinical parameters may assist clinicians to perform risk stratification in the emergency department and become another adjunct to the assessment of severe dengue.