The relationship between skeletal muscle mass at the beginning of the post-acute rehabilitation phase and rehabilitation outcomes has been scarcely investigated. The aim of this study was to investigate the impact of the existence of sarcopenia upon admission to a post-acute COVID-19 patient rehabilitation unit on body composition and functional and respiratory capacity at discharge. Thirty-four post-acute COVID-19 patients were referred to our Rehabilitation Unit from different COVID Hospitals in northern Italy. Body weight loss, body composition, handgrip strength, functional parameters, oxygen saturation and related perception of dyspnea in several positions were measured before and after a 28-day multidisciplinary rehabilitation program. Spirometry was performed only upon admission. The intervention included psychiatric support, cognitive behavioral therapy, nutritional therapy and physiotherapy, including aerobic and resistance training. Training volume was 45 min/session, 6 sessions/week. Upon admission, the prevalence of sarcopenia among our patients was 58%. In all of the 34 patients, we observed a trend of improvement in all of the respiratory, body composition, muscle strength and functional parameters considered. Monitoring muscle mass and strength in post-acute COVID-19 patients appears to be a key predictor of rehabilitation outcomes. Early diagnosis of sarcopenia therefore appears to be of paramount importance in the management of post-acute COVID-19 patients.