Background: Physical inactivity in older adults affects physical function, cognition, and social well-being. Rhodiola rosea, an adaptogenic herb, reported to improve endurance, reduce fatigue, and support mental health; however, clinical evidence in physically inactive older adults remains limited. Methods: This randomized, multicenter, double-blind, placebo-controlled study evaluated the efficacy and safety of Rhodiola rosea extract (RhoSera®) in 140 physically inactive adults aged 55 to 75 years, with early symptoms of muscle weakness or loss of muscle mass. The participants were equally randomized into two groups and observed for 12 weeks. Results: Compared with placebo, the Rhodiola rosea extract group showed improvements in anthropometric parameters, including waist and hip circumference, skinfold thickness, fat mass, muscle mass, lean body mass, and BMI. Improvements were also observed in handgrip strength, ASMI, balance, and postural stability, alongside reduced fatigue and stress, improved mood, and enhanced motivation for physical activity. Functional performance improved in the chair stand test, six-minute walk test, and gait speed. Conclusion: Rhodiola rosea extract is a safe and effective way to improve body composition, physical performance, and overall well-being in physically inactive older adults. These benefits may help preserve functional independence, mobility, and quality of life as people age.