Nanoindentation measurements of polygonal ferrite (PF), bainitic ferrite (BF) lath, ferrite in granular bainite (GB) and retained austenite (RA) in a low-alloyed multi-phase transformation-induced plasticity steel were carried out in conjunction with electron backscattering diffraction and scanning transmission electron microscopy (STEM). PF returned the lowest hardness followed by ferrite in GB, BF lath and RA. Dislocation glide and austenite-to-martensite transformation can be correlated with the first and second pop-in observed in the load–displacement curve, respectively. The martensite transformation induced by nanoindentation was demonstrated via correlative STEM. Film RA generally shows a higher stability than blocky RA, associated with a larger average onset load for martensite transformation in the former. It is noted that some ferrite in GB had similar hardness to BF lath and some blocky RA grains in GB showed a similar stability to that of film RA between BF lath, which can be attributed to a higher carbon content in GB due to chemical inhomogeneity.