The study investigated the effect of preoperatively diagnosed steatohepatitis associated with metabolic dysfunction (MASH) on weight loss outcomes after laparoscopic sleeve gastrectomy (LSG) in 226 obese patients who also underwent intraoperative liver biopsy. Preoperative body mass index (BMI) and presence of MASH were independent negative predictors of percent excess weight loss (%EWL) and percent total weight loss (%TWL) with P < 0.01. Postoperative %EWL differed by up to 28.9% between patients with and without MASH. Patients with preoperative MASH achieved a significantly lower cumulative incidence of 80% EWL within 1 year after surgery (χ² = 35.17, P < 0.05, HR = 2.058). Analyzes included univariate, multivariate and general linear models to compare weight loss trajectories, as well as Kaplan-Meier analysis. Preoperative MASH is significantly associated with lower postoperative weight loss and delay in achieving satisfactory weight loss results in the first year after surgery.