Background/Aims: To investigate the relation between skeletal muscle measurements (muscle mass, radiation attenuation, and sarcopenic obesity), postoperative morbidity, and survival after treatment of locally advanced rectal cancer. Methods: This explorative retrospective study identified 99 consecutive patients who underwent neoadjuvant chemoradiation and surgery between January 2007 and May 2012. Skeletal muscle mass was measured as total psoas area and total abdominal muscle area (TAMA) at 3 anatomical levels using the patient’s preoperative computed tomography scan. Radiation attenuation was measured using corresponding mean Hounsfield units for TAMA. Sarcopenic obesity was defined as body mass index above 25 kg·m–2 combined with skeletal muscle mass index below the sex-specific median. Postoperative complications were graded by using the Clavien-Dindo classification. Results: Twenty-five patients (25.3%) developed a grade 3–5 complication. Lower radiation attenuation was independently associated with overall (p = 0.003) and grade 3–5 complications (p = 0.002). Sarcopenic obesity was associated with overall complications (all p < 0.05). Skeletal muscle measurements and survival were not significantly related. Conclusion: Radiation attenuation was associated with overall and grade 3–5 postoperative morbidity after neoadjuvant chemoradiation and non-laparoscopic resection for rectal cancer. Sarcopenic obesity was associated with overall complications.