PurposePatients with obesity undergoing rectal cancer surgery may have an increased risk of developing complications, though evidence is inconclusive. The aim of this study was to determine the direct impact of obesity on postoperative outcomes using data from a large clinical registry.MethodThe Binational Colorectal Cancer Audit registry was used to identify patients who underwent rectal cancer surgery in Australia and New Zealand from 2007–2021. Primary outcomes were inpatient surgical and medical complications. Logistic regression models were developed to describe the association between body-mass index (BMI) and outcomes.ResultsAmong 3,708 patients (median age 66 years [IQR 56.75–75], 65.0% male), 2.0% had a BMI < 18.5 kg/m2, 35.4% had a BMI of 18.5–24.9 kg/m2, 37.6% had a BMI of 25.0–29.9 kg/m2, 16.7% had a BMI of 30.0–34.9 kg/m2, and 8.2% had a BMI ≥ 35.0 kg/m2. Surgical complications occurred in 27.7% of patients with a BMI of 18.5–24.9 kg/m2, 26.6% of patients with a BMI of 25.0–29.9 kg/m2(OR 0.91, 95% CI 0.76–1.10), 28.5% with a BMI of 30.0–34.9 kg/m2(OR 0.96, 95% CI 0.76–1.21), and 33.2% with a BMI ≥ 35.0 kg/m2(OR 1.27, 95% CI 0.94–1.71). Modelling BMI as a continuous variable confirmed a J-shaped relationship. The association between BMI and medical complications was more linear.ConclusionRisk of postoperative complications is increased in patients with obesity undergoing rectal cancer surgery.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00384-023-04447-0.