Background: Bariatric surgeries are a class of surgical procedures that are thought to be the safest and most efficient ways to treat obesity and the disorders associated with it. Aim: To evaluate the effect of educational guidelines on postoperative health outcomes among post-bariatric surgery patients. Design: a quasi-experimental research design was used (one group, pre-posttest). Setting: This study was applied in the Out-Patients Surgery Clinic at Damietta University Hospital. Sample: Included fifty post-bariatric surgery patients using a convenient sample. Tools: Tool I: patient assessment tool, Tool II: Malnutrition Universal Screening System (MUSS), Tool III: Complications assessment sheet, and Tool IV: International physical activity questionnaire. Results: Total post-bariatric surgery patients' knowledge level distribution as regards bariatric surgery pre and post-applying educational guidelines demonstrates that 16% of the post-bariatric surgery patients had a satisfactory level of total knowledge of pre applying educational guidelines. Compared to the intervention, this total knowledge level improved and became satisfactory among 95% of the post-bariatric surgery patients. A statistically significant difference was observed (P value = <0.001) and 4% of the patients had high levels of physical activity on the pretest, but 38% had high levels on the post-test. Conclusion: Applying educational guidelines has a positive effect on improving postoperative health outcomes among post-bariatric surgery patients. Recommendations: Provide post-bariatric surgery patients with a well-planned health education printed copy of educational guidelines to improve their awareness regarding bariatric surgery.