Objectives This study aimed to evaluate the effectiveness of de-training after a combined intervention of leucine-enriched whey protein supplementation and resistance training on skeletal muscle mass and strength in older adults with sarcopenia. Design A randomized controlled trial. Setting A community in Hyogo, Japan. Participants The study included older adults aged ≥ 65 years who were screened for sarcopenia at the Care Center from, a community in Hyogo, Japan. The participants were randomly allocated to one of three groups: whey protein supplementation after resistance training (RT + PRO; n = 27), resistance training only (RT; n = 27), and whey protein supplementation only (PRO; n = 27). Interventions An intervention program was conducted over a period of 24 weeks. The program included RT and/or PRO supplementation twice a week for 24 weeks, followed by a de-training period for another 24 weeks. PRO supplementation included 11.0 g of protein and 2,300 mg of leucine. The total energy and protein intake amounts for the participants in all groups were controlled to achieve at least 30 kcal/kg ideal body weight (IBW)/day and 1.2 g/kg IBW/day, respectively, during the intervention and de-training periods. Measurements The primary outcomes, such as mean change of appendicular skeletal muscle mass index (ΔASMI), and secondary outcomes, such as handgrip strength (ΔHGS), were measured at baseline, the end of intervention, and at 12 and 24 weeks of de-training. Results Compared to baseline, ASMI and HGS increased significantly at the end of the intervention period in the RT+PRO group (ASMI and HGS, p < 0.01); however, there were no significant differences in ΔASMI and ΔHGS between each group. At 24 weeks of the de-training period, ΔASMI and ΔHGS were higher in the RT + PRO group than in the RT group (p < 0.05 and p < 0.01, respectively). Conclusion We demonstrated that combined intervention of RT and PRO showed long-term maintenance in treating sarcopenia than RT only at 24 weeks after de-training. Therefore, PRO intake after RT may be useful in the treatment of sarcopenia in older adults.