Hip fractures are the most common osteoporotic fractures related to disability in older adults, requiring surgery and a subsequent rehabilitation treatment. Sarcopenia is currently considered as a predictive of worse outcome in hip fracture patients and myostatin has been recently proposed a potential biomarker of this condition. Twenty hip fracture patients after total hip replacement (mean aged 75.9 ± 2.4 years) were randomly divided into two groups of ten subjects (groups A and B). Both groups performed a rehabilitation program (5 sessions of 40 min/week for 2 weeks, followed by home-based exercise protocol). Group A received also 2-month amino acid supplementation. Serum myostatin levels significantly decreased after 2 months in both group A ( p  = 0.01) and group B ( p  = 0.03) in sarcopenic patients only in group A ( p  = 0.04). These results suggest that myostatin might be considered a promising biomarker of sarcopenia in hip fracture older adults’ patients undergoing rehabilitation and amino acid supplementation.