This article proposes a novel reflective-mode phase-variation sensor based on a movable step impedance resonator (SIR) implemented in coplanar waveguide (CPW) technology. The sensing principle is the variation of the phase of the reflection coefficient at a specific (operating) frequency generated by SIR motion. The sensor is modeled by a transmission line terminated with a series resonator connected to ground, and the reactive element subjected to motion-related variations is the SIR capacitance. The movable SIR, etched on an independent substrate, can be displaced either vertically or transversally, i.e., orthogonal to the symmetry plane of the CPW. In the first case, the device can be used as a highly sensitive proximity sensor with 10- μ m resolution. In the second case, the device can operate either as a linear or angular displacement and velocity sensor, provided that a linear or a circular chain of SIRs is etched in a movable substrate. The measurement of the (medium/long) range linear or angular displacement and velocity is based on pulse counting of the phase-modulated signal generated in the reflection coefficient by the motion of the SIR chain (transversely to the axis of the CPW). The presented prototypes, a micrometer-scale proximity sensor and a linear displacement/velocity sensor, validate the approach and point out the versatility of the proposed structure for motion sensing.