In this study, the characterisation of three-dimensional (3D)-printed substrates in terms of relative dielectric constant and loss tangent by using the T-resonator method is proposed. In particular, after the theoretical formulation of this method, the T-shape structure has been realised and its effectiveness in characterising also 3D-printed substrates in the frequency range between 800 and 5000 MHz has been experimentally demonstrated. Therefore, the T-resonator has been used to perform the microwave characterisation in terms of both dielectric constant and loss tangent of 3D-printed polylactic acid substrates when varying the infill percentage. Obtained results have been first summarised and discussed, and then used to realise an example of 3D-printed ultra-high-frequency radio-frequency identification tag, thus demonstrating the suitability of 3D-printable materials to be used as dielectrically controlled substrates for low-cost and high-performing electromagnetic applications. © The Institution of Engineering and Technology 2017.