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This study investigates the stress-induced formation of martensite during nanoindentation of an austenitic NiTi shape memory alloy, where stress-induced martensite is stable at room temperature. An individual grain with a [1 1 1] surface normal was selected for spherical ex situ and in situ nanoindentation in a scanning electron microscope. The in situ load-displacement curves show several pop-ins which occur concomitantly with the formation of traces around the contact zone between the indenter tip and the sample. These traces exhibit a threefold symmetry around the remnant indent. A detailed study of the indentation-induced surface relief by atomic force microscopy before and after shape recovery allows to identify the formation of six twinned martensite plates. Post-mortem microstructural characterization shows that these twinned martensite plates are growing as the applied load is increasing. The activation of the experimentally observed twinned martensite plates is rationalized by analytical calculations of resolved shear stress and mechanical interaction energy density. Finally, the in situ nanoindentation results in combination with the post-mortem microstructural characterization show that the most likely deformation mechanism responsible for pop-in events corresponds to sudden increases of the thicknesses of twinned martensite plates.