In this letter, a four-stage 235 GHz heterojunction bipolar transistor (HBT) amplifier is reported. Each stage uses a single-emitter 0.15 × 3 μm2 InP HBT with collector current density of 25 mA/μm2, maximum frequency of oscillation (fmax) greater than 500 GHz, and a cutoff frequency (fT) of 320 GHz. The MMIC amplifier is realized in inverted microstrip topology. The amplifier demonstrates measured small signal gain of 12.7 dB and output power of 2.6 mW with a power gain of 9.4 dB. This corresponds to transistor output power density of 5.7 mW/μm2 at 235 GHz. This is the highest HBT transistor output power density published to date at frequencies above 170 GHz, which indicates high power InP HBT power amplifiers should be viable with on-chip power combining.