Background Obesity disproportionately affects patients awaiting elective non-bariatric surgery and complicates perioperative management. This systematic review aimed to assess the impact of weight-loss interventions on intraoperative and postoperative outcomes. Methods MEDLINE, Embase, CINAHL, and Web of Science databases were searched from inception to October 2025 for trials on weight-loss interventions. Two reviewers independently screened the studies, extracted relevant data, and assessed risk of bias. Pooled mean differences (MDs), standardized mean differences (SMDs), and odds ratios (ORs) were obtained from random-effects meta-analyses. Results Thirty-five studies with 9378 participants (mean(standard deviation) age 58(8) years; body mass index 35.6(6.4) kg/m2; 61% women) were included. The median duration of intervention was 8 (interquartile range 4–14) weeks. Preoperative weight-loss interventions were significantly associated with a reduction in overall postoperative complications (odd ratio (OR) 0.63, 95% confidence interval (c.i.) 0.43 to 0.93; I² = 32%) and in complications requiring medical intervention graded as Clavien–Dindo ≥ II (OR 0.66, 0.51 to 0.86; I² = 0%). Additionally, they were linked to a decreased risk of postoperative non-infectious wound-related complications (OR 0.38, 0.15 to 0.97; I2 = 0%), and with reduced intraoperative blood loss in gastrectomy (SMD −0.98, 95% c.i. −1.47 to −0.48; I2 = 0%) and hepatectomy (SMD −0.41, −0.82 to 0.00; I2 = 0%). Reductions in blood transfusion (OR 0.49, 0.31 to 0.79; I2 = 0%), hospital readmission rates (OR 0.57, 0.47 to 0.70; I² = 0%), and length of hospital stay (SMD −0.08, −0.13 to −0.04; I2 = 0%) were also noted. No association was observed for surgical site infection, venous thromboembolism, or return to the emergency department. Compared with standard care or no intervention, weight-loss interventions led to greater weight loss (MD −3.92 (95% c.i. −5.44 to −2.39) kg; I2 = 91%), and fat mass loss (MD −4.78 (−6.49 to −3.06) kg; I2 = 0%) but no change in lean mass (SMD −0.25, −0.51 to 0.01; I2 = 0%). In a sensitivity analysis of studies at low risk of bias, the estimates and precision of most outcomes did not change materially. Conclusion Despite heterogeneity in study design and surgical populations, the evidence consistently demonstrated that weight-loss interventions are feasible, safe, and can reduce postoperative complications across various surgical specialties alongside improving many outcomes.