First experimental observations are reported on stimulated coherent synchrotron radiation from highly relativistic electrons in a strong magnetic field. The experiment employed a quasioptical millimeter-wave resonator and a 6-MeV electron beam gyrating in a field of up to 25 kG. Coherent radiation at 54 GHz, corresponding to the 11th gyroharmonic, was observed and characterized. These observations demonstrate the possibility of a synchrotron resonance maser.