This paper reports demonstration of a 0.666 THz (666 GHz) datalink. A peak error-free data rate of 9.5 Gbps is demonstrated over a 590 m range using QPSK modulation with a measured humidity of 18% with more than 40 dB SNR at the output of the receiver. The 0.666 THz link uses InP HEMT frequency converters providing both low noise and power amplification, a traveling wave tube Amplifier, and an advanced modem architecture. This ground-based demonstration indicates that traditional communications approaches can be used well into the submillimeter wave range. Moreover, a space-based crosslink based on this technology should be capable of transmitting Gbps-class data rates at LEO-LEO ranges.