Background: We aimed to investigate the association of body composition with survival and efficacy of first-line treatments in patients with castration-resistant prostate cancer (CRPC). Methods: Records of CRPC patients who were treated with first-line docetaxel or androgen receptor signaling inhibitors (ARSi) between 2005 and 2018 were reviewed. Skeletal muscle index (SMI), visceral fat index, and subcutaneous fat index were evaluated using pretreatment computed tomography images. Results: Of the 230 eligible patients, 144 patients received docetaxel and 86 received ARSi as the first-line treatment for CRPC. SMI^hi (based on median value) group had higher prostate-specific antigen (PSA) progression-free survival (median 13.5 vs. 8.3 months, p=0.030), radiologic progression-free survival (14.9 vs. 9.1 months; p<0.001), and overall survival (24.1 vs. 16.9 months, p=0.015) compared than SMI^lo group. In multivariable analysis, SMI was independently associated with risk of PSA progression (HR=0.68; 95% CI, 0.50-0.93; p=0.017), radiologic progression (HR=0.54; 95% CI, 0.39-0.75; p=0.001), and overall survival (HR=0.72; 95% CI, 0.52-0.98; p=0.037), regardless of BMI. In docetaxel-treated patients, PSA progression-free survival (13.5 vs. 5.9 months, p=0.016) and radiologic progression-free survival (14.6 vs. 6.7 months, p<0.001) were higher in SMI^hi than SMI^lo group. However, PSA progression-free survival and radiologic progression-free survival were similar between two groups in ARSi-treated patients. Based on treatment-specific hazards of radiologic progression, patients with SMI^lo are more likely to benefit from ARSi compared than docetaxel. Conclusions: High skeletal muscle mass may be associated with reduced risks of disease progression and mortality in patients with CRPC. However, the significance of these relationships is limited in patients treated with docetaxel. These results suggest that skeletal muscle mass may be helpful in treatment selection and predicting treatment response in CRPC. Further prospective studies are warranted.