2,3,7,8‐Tetramethoxythianthrene is known to crystallize in orthorhombic and monoclinic polymorphs, depending on the solvent used for crystallization. Upon heating the orthorhombic form to ca. 150°C, an enantiotropic (reversible) phase transition into the monoclinic phase occurs. This phase transition has been characterized by different techniques: differential scanning calorimetry (DSC), temperature‐resolved X‐ray powder diffractometry (TXRD), and thermally stimulated depolarization current measurements (TSDC). A mechanism for this solid‐state transition is proposed based on single‐crystal X‐ray structures.