This paper presents a 3D-printed dielectric directional coupler for the sub-THz band from 140GHz to 220GHz. In contrast to conventional coupling approaches, a quasi-optical operating principle is implemented, which is based on the insertion of a thin dielectric boundary within a dielectric image guide. Due to an intended permittivity shift, the boundary layer acts as a wavesplitter for incident HE₁₁ waves, whose partial reflections are selectively controllable via material or geometry parameters. This results in significant advantages in terms of achievable bandwidth and required dimensions, while manufacturing costs and complexity are minimized at the same time using additive manufacturing. The realized coupler exhibits broadband matching of approx. 20dB, with high isolation of 25dB and achieves freely configurable, reproducible broadband coupling attenuation between 5dB and 15dB. Due to the remarkable simplicity and small size of the concept, this demonstrates a promising key component for low-loss, large-scale, dielectric sub-THz signal distribution networks.