Melatonin, a hormone produced in the pineal gland, has been investigated for bone repair, remodeling, osteoporosis, as well as osseointegration of the implants. In this study, different concentrations of melatonin (0-2000-mu M) were embedded into silk films annealed by methanol or water. Then, their capacity to differentiate human mesenchymal stem cells into osteoblasts was investigated for bone tissue regeneration. While methanol-annealed silk films have similar to 55% crystallinity, room-temperature water-annealed silk films have similar to 30% crystallinity by depending upon their different beta-sheet contents. Melatonin-loaded silk films exhibited an initial burst release followed by a continuous release for up to 5 days, and the beta-sheet content of silk films did not affect the release behavior of melatonin, an amphiphilic molecule. Moreover, human mesenchymal stem cells exhibited an increase in osteogenic markers such as alkaline phosphatase activity, osteocalcin, and runt-related transcription factor 2 expressions on the melatonin-loaded methanol-annealed silk films in both proliferation and osteogenic media. The bioactivity of the melatonin-modified silk films was further confirmed by the enhanced mineralization compared to silk films alone. This study demonstrated the feasibility of developing melatonin-loaded silk materials and the positive effect of releasing melatonin at micromolar concentrations on osteogenic differentiation of human mesenchymal stem cells cultured especially in osteogenic medium.