Biologists introduced some years ago an intermediate biological model between 2D cell culture and 3D in vivo tissues, called microtissues or spheroids. To study such 3D models, techniques are currently applied, including electromagnetic waves-based methods at low frequencies. This frequency range does not allow the penetration of the waves inside the cells, conversely to the microwaves. There is consequently a strong interest to develop an appropriate microwave-based sensor dedicated to microtissues analysis. This constitutes the topic of this paper with the introduction of a new microwave sensor, suitable for the dielectric characterization of microtissues. An analytic method is proposed to extract their electrical parameters. The evaluation of the biosensor is performed, while measuring large polystyrene beads with known dielectric properties, followed then by the dielectric evaluation of fixed spheroids from 500MHz to 20GHz.