Background: Adult childhood cancer survivors are at risk for frailty, including low muscle mass and weakness (sarcopenia). Using peripheral blood mitochondrial DNA copy number (mtDNAcn) as a proxy for functional mitochondria, this study describes cross-sectional associations between mtDNAcn and sarcopenia among survivors. Methods: Among 1762 adult childhood cancer survivors (51.6% male; median age = 29.4 years, interquartile range [IQR] = 23.3-36.8), with a median of 20.6 years from diagnosis (IQR = 15.2-28.2), mtDNAcn estimates were derived from whole-genome sequencing. A subset was validated by quantitative polymerase chain reaction and evaluated cross-sectionally using multivariable logistic regression for their association with sarcopenia, defined by race-, age-, and sex-specific low lean muscle mass or weak grip strength. All statistical tests were 2-sided. Results: The prevalence of sarcopenia was 27.0%, higher among female than male survivors (31.5% vs 22.9%; P <. 001) and associated with age at diagnosis; 51.7% of survivors with sarcopenia were diagnosed ages 4-13 years (P =. 01). Sarcopenia was most prevalent (39.0%) among central nervous system tumor survivors. Cranial radiation (odds ratio [OR] = 1.84, 95% confidence interval [CI] = 1.32 to 2.59) and alkylating agents (OR = 1.34, 95% CI = 1.04 to 1.72) increased, whereas glucocorticoids decreased odds (OR = 0.72, 95% CI = 0.56 to 0.93) of sarcopenia. mtDNAcn decreased with age (β = -0.81, P =. 002) and was higher among female survivors (β = 9.23, P =. 01) and among survivors with a C allele at mt.204 (β = -17.9, P =. 02). In adjusted models, every standard deviation decrease in mtDNAcn increased the odds of sarcopenia 20% (OR = 1.20, 95% CI = 1.07 to 1.34). Conclusions: A growing body of evidence supports peripheral blood mtDNAcn as a biomarker for adverse health outcomes; however, this study is the first to report an association between mtDNAcn and sarcopenia among childhood cancer survivors.