Objectives Weight loss is a key component in the management of type 2 diabetes mellitus (T2DM). This outcome can be potentially achieved by laparoscopic sleeve gastrectomy (LSG). This study aims to determine the efficacy of LSG in reducing weight and to evaluate its impact on controlling T2DM by measuring HbA1c and body mass index (BMI) preoperatively and during the first year following the procedure. Methods This retrospective study was carried out on 340 patients aged ≥15 years who had T2DM and a BMI of ≥ 35 kg/m 2 . These patients had LSG at King Fahad Hospital Almadinah Almunawwarah KSA between January 2015 and July 2019. Their HbA1c and BMI were measured preoperatively and then postoperatively at less than one month, 1–3, 4–6, 7–9, and 10–12 months as well as after one year. Results Average BMI dropped consistently from a preoperative BMI of 49.27 kg/m 2 to 32.72 kg/m 2 at 10–12 months following LSG. A reduction in HbA1c from 8.38% to 6.43% was observed over one year ( p = 0.0001). Seventy-five percent of the patients achieved the HbA1c target of 6.5% or less within one year. The remaining 25% of the patients showed improvement in their HbA1c but did not reach the target level. Conclusion This study endorses a positive impact of LSG on both weight loss and diabetic status. There was a significant reduction of both BMI (up to the first year) and HbA1c levels postoperatively.