We present the experimental demonstration of the first terahertz isolator (working frequency > 1 THz), obtained using the non-reciprocal electromagnetic properties of graphene under magnetostatic bias. The presentation will cover in detail the design, fabrication and measurement of the device. The isolator operated in reflection for circularly polarized waves, showing two bands at 3 and 7.5 terahertz respectively. In spite of the simplicity of design and fabrication, the proposed device geometry achieves near-optimal performances. We believe these results to be groundbreaking and of great importance for the terahertz community.