Ni-Mn-Ga alloy system has attracted considerable attention during the past few years and is expected to become a promising potential for fast-responsive and compact actuators owing to its large magnetic field induced strain (upto 10%) along with other interesting properties like premartensitic transition, large magnetocaloric effect, high transformation temperature and low stress for twin variant rearrangement. Depending upon the composition, the most important martensites that exist in off-stoichiometric Ni₂MnGa alloys are the modulated 5M and 7M, and, the non-modulated tetragonal NM ones. In this study, a systematic investigation was carried out on the 5M martensite in a well characterised Ni₅₀Mn_28.9Ga_21.1 alloy using transmission electron microscopy (TEM) and x-ray diffractometry for determining its crystal structure, substructure, intervariant interfaces and self-accommodating microstructure. It was established that the 5M martensite is a monoclinic crystal structure with internally faulted substructure. Figs1-3. The crystallographic features of the martensitic microstructure as observed by TEM are found to be in agreement with those predicted by computations based on the phenomenological theory of martensite crystallography.