Reported here is the fabrication and characterization of a three-dimensional photonic crystal for terahertz frequencies based on 3D printing of Al₂O₃. Specific inks containing Al₂O₃, the material required for the structure, are synthesized with the viscosity and rheological properties necessary to be extruded through nozzles of micrometric section. The process is completed by a thermal sintering to obtain compact structures. SEM measurements are performed to evaluate the quality of the structures and measure the geometrical parameters. Finally, the presence of the band gap is demonstrated by THz time-domain spectroscopy.