This study presents a dual-band power amplifier (PA) with two output ports using a simplified three-port, frequency-dividing matching network. The dual-band, dual-output PA could amplify a dual-band signal with one transistor, and the diplexer-like output matching network (OMN) divided the two bands into different output ports. A structure consisting of a λ/4 open stub and a λ/4 transmission line was applied to restrain undesired signals, which made each branch equivalent to an open circuit at another frequency. A three-stub design reduced the complexity of the OMN. Second-order harmonic impedances were tuned for better efficiency. The PA was designed with a 10-W gallium nitride high electron mobility transistor (GaN HEMT). It achieved a drain efficiency (DE) of 55.84% and 53.77%, with the corresponding output power of 40.22 and 40.77 dBm at 3.5 and 5.0 GHz, respectively. The 40%-DE bandwidths were over 200 MHz in the two bands.