<p>Martensitic transitions in organic crystals, which are diffusionless structural phase transitions between polymorphs, are topics of current interest. These transitions proceed by fast propagation of the phase front (phase boundary) across the crystals and induce fascinating phenomena, such as thermosalient effect and superelasticity, which have potential applications in molecular devices. Revealing the mechanism of these transitions is of crucial importance for understanding the origins of these phenomena and for their device applications. To this end, here we observe the spatio-temporal evolution of the phase front in temperature-induced martensitic transition in 1,2,4,5-tetrabromobenezen (TBB) crystals. We observed subsonic phase front propagation across the crystals with the velocity of up to ~800 m/s. The analysis using a nonlinear lattice model based on Landau’s theory of phase transitions showed that the phase front motion is described as a propagating solitary wave in a non-conservative system. Our results revealed the essential factors governing organic martensitic transitions and novel insights into the transition mechanism.</p>