A 300 GHz transmitter (TX) fabricated using a 40 nm CMOS process is presented. It achieves 17.5 Gb/s/ch 32-quadrature amplitude modulation (QAM) transmission over six 5 GHz-wide channels covering the frequency range from 275 to 305 GHz. With the unity-power-gain frequency of the NMOS transistor being below 300 GHz, the TX adopts a power amplifier-less QAM-capable architecture employing a highly linear subharmonic mixer called a cubic mixer. It is based on and as compact as a tripler and enables the massive power combining necessary above without undue layout complication. The frequency-dependent characteristics of the cubic mixer are studied, and it is shown that even higher data rates of up to 30 Gb/s are possible at certain frequencies, where the channel signal-to-noise ratio is high. The design and the operation of the power-splitting and power-combining circuits are also described in detail. The measurements reported herein were all made “wired” via a WR3.4 waveguide.