This study was performed to quantify the morphological characteristics of the psoas major muscle in patients with symptomatic multilevel degenerative lumbar spinal stenosis (SMLSS) and assess the correlations of these morphological characteristics with function and clinical symptoms.One hundred fourteen patients diagnosed with SMLSS (≥ 3 segments) were included. The patients' presenting symptoms were assessed with the Oswestry Disability Index (ODI), and visual analogue scale (VAS) scores were recorded. The morphology of the psoas major was evaluated at the L3/4 intervertebral disc level in three ways: by measuring (i) the psoas muscle mass index (PMI); (ii) the mean muscle attenuation (Hounsfield units, HU); and (iii) the morphologic change of the psoas major (mean ratios of the short axis to the long axis of the bilateral psoas major).Men had a higher PMI than women (p = 0.001). Patients with severe disability had a significantly lower PMI (p = 0.002) and muscle attenuation (p = 0.001). The PMI and muscle attenuation were significantly higher in the patients with no or mild back pain (both p < 0.001). In the univariable and multivariable analyses, a greater HU value was associated with a higher functional status as assessed by the ODI (p = 0.002), and a higher PMI was associated with less severe back pain as measured by the VAS score (p < 0.001).This study showed that muscle attenuation of psoas major positively correlated with the functional status and PMI negatively correlated with low back pain severity in patients diagnosed with SMLSS. Future prospective studies are needed to evaluate whether improvement in such muscle parameters through physiotherapy programs can alleviate the clinical symptoms and improve the functional status of patients with SMLSS.© 2023. The Author(s).