This article presents a gallium nitride (GaN) wideband millimeter-wave power amplifier (PA) incorporating the Chebyshev matching technique. The theoretical design method of the wideband $N$ -order Chebyshev matching network is proposed. Considering the insertion loss and circuit complexity, the second-order Chebyshev network is designed, which is implemented by transmission lines (TLs) and capacitors. Based on the designed matching network, a $K$ -band PA is designed. Fabricated in a 250-nm GaN process, our PA scores the highest in-band gain of 23.8 dB at 23.6 GHz, 28% fractional bandwidth across 18.5–24.5 GHz, 32% peak power added efficiency (PAE), and 4.5-W saturated output power. The power density is 0.96 W/mm $^{2}$ and the chip area is 2.4 $\times$ 1.95 mm $^{2}$ .