In a thermoelastic martensitic transformation, there is a “self-accommodation” in which the microstructure itself relieves strain induced by the transformation. In this study, the crystal structure and self-accommodation microstructure of martensite in Ti₃₀Ni₅₀Zr₂₀ alloy were investigated using X-ray diffraction, scanning electron microscopy with electron backscatter diffraction, and transmission electron microscopy. In addition, the deformation microstructure was investigated by observing the samples after tensile tests. The crystal structure of the martensite in the Ti₃₀Ni₅₀Zr₂₀ alloy was determined to be the B19′ monoclinic structure. In this alloy, plate- and polygonal-like variants with a width of a few microns were observed, and pairs of habit-plane variants (HPVs) forming {011}_B19′ twins were observed at the interface. The self-accommodation has a mosaic-like morphology because of the combination of these pairs of HPVs. The (001)_B19′ compound twins were formed as internal defects. These twins are considered to be lattice-invariant shear (LIS) of martensite in this alloy. No plateau region was observed in the tensile test. In the deformed specimen, the self-accommodation was collapsed by the movement of the HPV interface and the introduction of (100)_B19′ compound twins was observed as an internal defect. This defect is considered to be not LIS but a deformation twin. The lack of a plateau region in the stress–displacement curve was attributed to these deformed microstructures.