We propose a compact, high-power, and highdirectivity surface-emitting terahertz (THz) source based on an array of active antennas with integrated patch antennas and resonant-tunneling diodes (RTDs). An array configuration of active antennas, each with an integrated patch antenna and two RTDs and coupled by microstrip lines, enables spatial power combining and improves directivity through coherent oscillation. We confirmed a maximum radiation power of 11.8 mW in a prototype 6 × 6 array at an oscillation frequency of 0.45 THz. Parasitic oscillation at low frequencies was suppressed by use of a bias stabilization circuit consisting of series-connected resistors and capacitors, and the dc to RF efficiency of this device was estimated to be approximately 1%. The radiant intensity of 210 mW/sr and the 3-dB beamwidth of 13°for the measured 6 × 6 array confirmed that directivity was improved by coherent oscillation based on mutual injection locking. The directivity of the patch antenna capable of surface emission can be controlled only by the number of antennas, even if there is no secondary radiator, such as an Si lens. The obtained results suggest that RTDs are promising as practical THz sources for realizing applications of THz imaging and 6G communication.