The first-order phase transformation in NaCN at 288 °K is identified as a martensite transformation accompanied by the formation of mechanical deformation twins of the first kind. Photomicroscopy measurements of the crystal surface morphology and nuclear magnetic resonance measurements of the domain orientation are in agreement with the predictions of the Weschsler, Lieberman, and Read theory of twin formation accompanying a martensite transformation. There are 24 domains; the group S2 leaves a domain invariant and the domain generating group of F operations is either O or Td. NaCN is identified as belonging to the ferroelastic species m3mF1̄.