In this paper, an out-of-band suppression enhanced high-efficiency millimeter-wave (mm-wave) gallium nitride (GaN) microwave monolithic integrated circuit (MMIC) filtering power amplifier (FPA) is presented. A modified elliptic response bandpass filtering (BPF) output matching network (OMN) is used to achieve low loss and high out-of-band rejection. The circuit of the presented BPF OMN is given and its synthesized method is introduced. The simulated S-parameters show that the OMN has a low-loss in-band response and a desired out-of-band rejection performance with two transmission zeros (TZs). To demonstrate this idea, a two-stage 24–28-GHz GaN MMIC PA is designed and implemented in a 0.15-μm GaN-on-SiC process, with a mask area of 2.4 × 1.0 mm². Experimental results show that the realized MMIC PA achieves a small signal gain of 18.8 dB from 24 to 28 GHz with ±1.2-dB gain flatness and 40-dB rejection at 19.43 and 33.8 GHz. The saturated power-added efficiency (PAE) is higher than 29% from 24 to 28 GHz, with a peak PAE of 36.3% at 27 GHz. The output power is within the range of 33.8 to 35.6 dBm in the operating band. For a 100-MHz five-carrier LTE signal, the PA can achieve ACLR better than −40 dBc over the band with digital predistortion.