<p>Visual, power-free temperature sensors capable of operating near the boiling point of water are highly desirable for safety and warning applications, yet reusable systems in this temperature regime remain limited. Here, we report a reversible colorimetric temperature sensor based on polydiacetylene (PDA) embedded in a polystyrene (PS) matrix for the visual detection of near-boiling liquids. The sensor exhibits a distinct blue-to-red color transition upon exposure to near-boiling water and spontaneously recovers to its original blue state upon cooling, allowing repeated use without external power or instrumentation. The switching behavior is closely associated with the glass transition temperature of the PS matrix, which was measured to be 101.8&#xa0;°C. Quantitative solid-state UV–Vis spectroscopy was used to define the colorimetric response and confirm reproducible cycling behavior. Differential scanning calorimetry correlated the sensing window with the glass transition behavior of the PS matrix, while Fourier-transform infrared spectroscopy showed no clear evidence of irreversible chemical degradation during reversible chromatic switching. By combining a chromatic conjugated polymer with polymer-matrix-assisted thermal programming, this work establishes a simple and reusable colorimetric temperature sensor operating in the near-boiling regime, offering a practical strategy for passive thermal warning and consumer safety applications.</p> Graphical Abstract <p></p>