This letter presents a fully integrated Class-J GaN monolithic microwave circuit power amplifier (PA), which is fabricated in Woolfspeed 0.25- $\mu \text{m}$ GaN-on-SiC technology. This PA is the first published design for the emerging IEEE 802.11ax application in the literature. When tested with 80-MHz 256-quadratic-amplitude modulation 802.11ax signal with 11.25-dB peak-to-average power ratio, the PA delivers average output power of 27.3–30.3 dBm from 4.9 to 5.9 GHz, with power-added efficiency of 16.7% to 27.3%, while meeting the standard specification of error vector magnitude below −32 dB.