The vapochromic single‐crystal‐to‐single‐crystal (SCSC) transformation of a highly luminescent PtII complex bearing an N‐heterocyclic carbene Pt(CN)2(tBu‐impy) (tBu‐impyH+=1‐tert‐butyl‐3‐(2‐pyridyl)‐1H‐imidazolium) is reported. The trihydrate form of the complex, which exhibits blue 3MMLCT emission owing to weak Pt⋅⋅⋅Pt interactions, changed its luminescence color from blue to yellowish‐green upon the desorption of water molecules while keeping the high emission quantum yield of more than 0.45. Variable‐temperature and continuous in‐situ tracking of single‐crystal X‐ray diffraction measurements revealed that the SCSC transformation proceeds reversibly by the release and reabsorption of water molecules, thereby changing the stacked structure slightly. As a result, the dynamics of vapor‐induced SCSC transformation were elucidated: that the anhydrous form returned to the original trihydrate form in a two‐step process under a water vapor atmosphere. In addition, the PtII complex exhibited a similar SCSC response accompanied by a luminescence color change in the presence of methanol vapor, while being inactive toward ethanol vapor. Out of the blue: The blue color of the luminescence of a PtII complex bearing an N‐heterocyclic carbene changes in response to the vapor atmosphere, although the single crystallinity and luminescence intensity are maintained. The vapor‐induced reversible and stepwise single‐crystal‐to‐single‐crystal (SCSC) transformations were elucidated by variable‐temperature and continuous in‐situ tracking of single‐crystal X‐ray diffraction measurements.