Microwave dielectric spectroscopy of biological materials is gaining interest as it presents attractive features such as being noninvasive, rapid, label-free, and suitable for intracellular investigations, notably for biological and medical researches. Specific microwave biosensors have consequently been developed to investigate biomolecules, cells in suspension, and also tissues. A type of biosensor is, however, missing for the analysis of 3-D aggregates of cells, also called microtissues or spheroids, which constitute an intermediate biological model between 2-D cell cultures and tissues or organs. Therefore, we present a microwave-based microsensor suitable for the study of such 3-D biological models. It is composed of a coplanar waveguide (CPW) with a central capacitive gap, which allows to measure the capacitive and conductive contrasts between the studied bio-object and a reference liquid, typically the cell culture medium, in the frequency range from 500 MHz to 20 GHz. This sensor also integrates a microfluidic channel to maintain the microtissues in liquid. Two fluidic configurations are investigated, one open and one closed, the latter including a mechanical trap. The advantages of the closed configuration are then highlighted experimentally, enabling accurate and real-time microwave measurements of microtissues maintained in their culture medium and in the vicinity of the sensing area for sensitivity enhancement. In addition, an analytical study on the capacitive gap size performed with both simulations and experiments leads to an optimal gap size for sensitivity improvement. This new sensor consequently offers new noninvasive, nondestructive, and dielectric sensing possibilities for biomedical investigations such as drug screening, personalized medicine, and also fundamental research.