This paper presents a millimeter-wave (127 GHz) CMOS transceiver with a digital pre-distortion capable PAM-4 modulator for contactless communications. The transmitter upconverts PAM-4 modulated baseband signals through a free-running 127-GHz oscillator and single-balanced mixer, and it delivers PAM-4 modulated carrier signals to a folded-dipole antenna, which is designed on a FR408HR substrate. The receiver's low-noise amplifier provides a 10-dB gain, and the self-mixer downconverts carrier-modulated PAM-4 signals to baseband signals without the necessity of carrier synchronization. The PAM-4 modulator pre-distorts the baseband signals and corrects the non-linear characteristics of the transmitter's upconversion mixer and the receiver's downconversion self-mixer. Designed and fabricated in a 65-nm CMOS process, the demonstrated system transfers 20 Gb/s of PAM-4 modulated data through a 1-mm air gap and consumes 79.5 mW (transmitter: 50.8 mW and receiver: 28.7 mW) of power under a 1.2-V supply, achieving a 3.98-pJ/bit energy efficiency. The communication distance is extended to 3 cm by inserting a dielectric waveguide between the same transceiver.