Background/Objectives: Adult degenerative scoliosis (ADS) is a three-dimensional spinal deformity caused by asymmetric degeneration of intervertebral discs, facet joints, and paraspinal muscles (PSM). Although previous studies have identified bilateral asymmetry in the multifidus (MF), the erector spinae (ES), and psoas (PS), the underlying mechanism remains unclear. This study aims to compare muscle volume (MV), fat infiltration (FI), and degenerative changes on the concave (CC) and convex (CV) sides in symptomatic ADS patients. Methods: This study included patients with ADS who visited Kangbuk Samsung Hospital between 2022 and 2024. MV and FI were measured by CT scans, and facet joint degeneration (FJD), disc degeneration (DD), and foraminal stenosis (FS) were assessed by MRI. The values of CV and CC side were compared. Results: 74 patients were enrolled with a mean age of 71.38 ± 8.6 years. MV of all PSM was significantly lower on the CC side than CV side. FI of ES and MF was significantly higher on the CC side compared to the CV side. Degenerative changes in the intervertebral discs and neural foramen were more severe on the CC side. Conclusions: In patients with ADS, we found that MV was consistently lower on the CC side across all PSM, whereas FI in the MF and ES was higher on the CC side. Degenerative changes in the spine, such as FS and DD, were also more severe on the CC side, although FJD was not. These results suggest that spinal muscle and structural degeneration are not evenly distributed between two sides. Physiotherapy that specifically targets the more affected CC side may offer better outcomes for patients with ADS.