Abstract: Star–planet magnetic interaction (SPMI) is an important physical process through which close-in planets and their host stars exchange energy via magnetic coupling. When a planet orbits within the sub-Alfvénic region of the stellar wind, perturbations of the surrounding magnetized plasma induced by the relative motion between the planet and the stellar wind can propagate along stellar magnetic field lines toward the host star, potentially producing enhanced activity in the stellar atmosphere modulated by the planetary orbital motion. Such signatures provide a promising means of probing the magnetic coupling between stars and planets, although their identification is often complicated by intrinsic stellar magnetic activity and the temporal variability of the strength of the interaction. This review summarizes the major observational advances, theoretical models, and potential applications of SPMI. Observationally, we review candidate SPMI signatures identified in chromospheric spectral lines, optical photometry, ultraviolet and X-ray observations, and radio observations. These candidate signatures include orbital-phase-dependent variations in chromospheric emission, possible clustering of stellar flares at particular orbital phases, enhanced high-energy emission, and coherent radio bursts. Particular attention is paid to their orbital-phase dependence, temporal variability, and the difficulty of distinguishing planet-induced signals from stellar rotation, active-region evolution, and intrinsic flaring activity. Theoretically, we discuss four major frameworks, including the Alfvén wing model, the star–planet magnetic reconnection model, the planet-triggered coronal energy-release model, and the star–planet interconnecting flux-tube model. We compare their underlying physical mechanisms and energy budgets, as well as their consistency with currently reported observations. The strength and observable manifestations of SPMI predicted by these models are sensitive to the stellar wind environment, magnetic-field configuration, planetary magnetic properties, and relative motion between the planet and the surrounding plasma. Three-dimensional magnetohydrodynamic simulations provide an important tool for characterizing the Alfvénic environment of close-in planets and evaluating the associated energy transport. SPMI studies not only contribute to our understanding of the two-way magnetic coupling between stars and planets, but also provide new approaches for searching for unknown close-in exoplanets, constraining exoplanetary magnetic fields, and evaluating the effects of stellar activity on planetary atmospheric evolution and habitability. At present, however, most reported SPMI signatures remain candidates, and establishing robust detections requires repeated observations over multiple planetary orbits, preferably with complementary multiwavelength diagnostics, as well as careful separation of planet-induced signals from intrinsic stellar variability. In the future, long-term time-domain surveys, high-precision spectroscopic observations, and next-generation radio facilities, combined with stellar magnetic-field measurements and three-dimensional numerical simulations, are expected to provide more stringent observational tests of SPMI. These advances will help establish more accurate physical models of magnetic star–planet coupling and improve our understanding of the long-term evolution of star–planet systems and planetary habitability.