A device for adjusting the phase angle of a camshaft of an internal combustion engine includes a drive gear for driving a camshaft accommodated in a coaxial arrangement relative to the camshaft and an electric motor communicating with the camshaft via a Harmonic Drive having a roller bearing with an elliptical inner ring, an externally toothed, flexible gear arranged on the roller bearing, and a rigid, internally toothed gear engaging the externally toothed gear. A simple structure is achieved by providing the electric motor with a housing which is rigidly connected to the camshaft via retaining elements which extend through recesses in the drive gear so that the recesses restrict the allowable adjusting range of the camshaft.