We report that topological features of the dynamic electron density provide a microscopic electronic order parameter that directly reflects the extent of structural change during a phase transition and offers insight into its underlying mechanism. Using multipole electron-density models derived from extensive high-resolution synchrotron X-ray diffraction datasets collected on α-KNO3 at 100 K, 200 K, 290 K and 380 K, we reconstruct the thermally convoluted dynamic electron density at each temperature. It is found that its temperature evolution quantitatively captures the progressive suppression of local symmetry breaking as the system approaches the α→β phase transition. The topological descriptors of the electron density associated with the nitrate group exhibit a non-monotonic temperature evolution with the initial inequivalence among the N–O bond electron densities undergoing an order-parameter-like collapse upon heating to 380 K, preceding the actual phase-transition temperature. These findings demonstrate that dynamic electron-density topology can provide valuable microscopic insight into structural phase transitions.