We investigated the emission of subterahertz frequency electromagnetic radiation from high-TC superconducting c-axis NdBa2Cu3O7−8/PrBa2Cu3O7−8/NdBa2Cu3O7−8 trilayer thin film tunneling junctions when external electric and magnetic fields are applied. The current-voltage characteristics under applied ab-plane magnetic fields H (up 8 T) exhibit well defined steps, Vn, such that eVn -hwp / (2n), where the plasma frequency wp-0.4 THz and n=1,2,3, . . . .. These steps may be interpreted using Josephson plasma dynamics. The applied voltage creates oscillating currents via the Josephson coupling energy EJ (EJ=hIJ /4e, where IJ is the Josephson current and h is Planck’s constant) and the charge energy EC (EC=e2 /2C, where C is the junction capacitance). Thus the Josephson plasma becomes excited by the tunneling current, with some of the energy being emitted as subterahertz frequency radiation. Our results provide a new insight into a solid-state quantum system with considerable potential for new solid-state terahertz emission sources.