This letter introduces a novel millimeter (mm)-wave oscillator concept based on an electromagnetic bandgap (EBG) resonator (also called a photonic crystal resonator). To realize an ultralow phase noise monolithic microwave integrated circuit (MMIC) oscillator, an EBG resonator is developed by introducing a periodic structure into an ultrahigh resistivity silicon wafer to create an EBG which confines a localized resonant mode with a reduced mode dielectric filling factor of 47%. The measured results of the resonator demonstrate an unloaded Q-factor of 108300 can be achieved at 45 GHz. Measured oscillator phase noise levels of −91.5, −121.5, and −133 dBc/Hz are obtained at offset frequencies of 1, 10, and 100 kHz, respectively.