This paper presents a high-power high-efficiency stacked D-band power amplifier (PA) in a 250-nm HBT technology. The pseudo-differential PA is 2-way power-combined with a power combining network exhibiting a single-transformer footprint and a low loss by reflecting asymmetry leaking to the common mode. The presented PA has a saturated output power of 20.9 dBm and a peak power-added efficiency (PAE) of 24.3%. A small-signal gain of 14.2 dB is obtained with a 3-dB bandwidth of 19 GHz.