Dielectric reflectarray antennas are proposed as a promising low-loss and low-cost solution for high gain terahertz (THz) antennas. Variable height dielectric elements are used in the reflectarray designs, which allow for the use of low dielectric-constant materials. Polymer-jetting 3-D printing technology is utilized to fabricate the antenna, which makes it possible to achieve rapid prototyping at a low-cost. Numerical and experimental results are presented for 3 different prototypes operating at 100 GHz, which show a good performance. Moreover the methodology proposed here is readily scalable, and with the current material and fabrication technology, designs up to 1.0 THz can be realized. This study reveals that the proposed design approach is well suited for low-cost high-gain THz antennas.