This article proposes a highly sensitive single-frequency microwave sensor for dielectric characterization of solids and liquids. The sensor is a reflective-mode one-port device consisting of a step-impedance microstrip line terminated with a slot resonator (the sensing element) transversely etched on the ground plane. The working principle of the sensor is the variation experienced by the phase of the reflection coefficient at the operating frequency as a consequence of changes in the dielectric properties (dielectric constant) of the material under test (MUT), either a solid or a liquid, which should be in contact with the slot resonator. A relevant advantage of the reported sensor is the fact that the MUT is located in the back substrate side of the microstrip structure. Therefore, any potential interaction between the MUT, including the mechanic holder used for liquid sensing, and the microstrip line, is prevented. A detailed sensitivity analysis that considers the effects of losses is carried out. It is concluded that, depending on the losses of the MUT, there are two different sensor operation regimes (the low-loss and the high-loss regimes), with unequal behavior, but both are useful to sensitively detect changes in the dielectric properties of the MUT. The maximum sensitivity in the prototype device devoted to the measurement of the dielectric constant in low-loss solid samples is 255.5 ∘. In the liquid sensor, equipped with a 3-D-printed holder, and focused in this article on the detection of small volume fractions of isopropanol in deionized (DI) water, the maximum sensitivity is 64.96 ∘ /%.