This article presents a two-way J-band PA with a flat-gain response and high saturated output power in SiGe BiCMOS technology with experimental ƒ_T and ƒ_max of 470 and 650 GHz, respectively. The PA design features three cascaded amplifier stages with interstage matching achieved by the passive distribution and combining networks. The fabricated PA demonstrated a maximum collector efficiency of 5.5% at a saturated output power of 15.5 dBm. Small-signal gain of 20 dB was achieved with a flat gain response of ±0.6 dB from 255 to 315 GHz. The PA is well suited for J-band transmitting arrays with λ/2 spacing for communication and sensing applications.