Bariatric surgery (BAR) has become a popular treatment for type 2 diabetes mellitus (T2DM) which is among the most critical obesityrelated comorbidities. Patients who have bariatric surgery, are exposed to complications after surgery. Furthermore, the mid-to longterm complications after bariatric surgery can be deadly and increase the complexity of managing safety of these operations and healthcare costs. Current studies on BAR complications have mainly used risk scoring for identifying patients who are more likely to have complications after surgery. Though, these studies do not take into consideration the imbalanced nature of the data where the size of the class of interest (patients who have complications after surgery) is relatively small. We propose the use of imbalanced classification techniques to tackle the imbalanced bariatric surgery data: synthetic minority oversampling technique (SMOTE), random undersampling, and ensemble learning classification methods including Random Forest, Bagging, and AdaBoost. Moreover, we improve classification performance through using Chi-Squared, Information Gain, and Correlation-based