<p>Sarcopenia diagnosed via opportunistic computed tomography (CT) is linked to poor postoperative outcomes. This study aimed to investigate the association between chest CT-diagnosed sarcopenia and post-total knee arthroplasty (TKA) functional outcomes. This single-center retrospective cohort study included 158 knee osteoarthritis (KOA) patients who underwent primary TKA between January 2021 and June 2022. Sarcopenia was defined using skeletal muscle index values derived from chest CT images at the T12 vertebral level. Sociodemographic, clinical, and perioperative data were collected, and functional outcomes were assessed via the Hospital for Special Surgery (HSS) score at 6 months postoperatively. Univariate and multivariate analyses identified independent risk factors for poorer postoperative functional outcomes in KOA patients undergoing TKA. The prevalence of sarcopenia in the cohort was 48.7%. At 6 months after TKA, sarcopenic patients had significantly lower HSS scores (<i>P</i> &lt; 0.05). Advanced age (<i>P</i> = 0.025), higher body mass index (<i>P</i> = 0.027), sarcopenia (<i>P</i> = 0.020), and higher postoperative pain scores (<i>P</i> &lt; 0.001) were independently associated with poorer functional outcomes. The results indicate that chest CT-determined sarcopenia is prevalent among KOA patients undergoing TKA and is significantly associated with poorer functional outcomes.</p>