As-built Laser Powder Bed Fusion (LPBF) Ti-6Al-4V typically exhibits a fully acicular alpha '-martensite microstructure, and requires post-process heat treatment in order to decompose the martensite and achieve sufficient ductility. In the present study, we demonstrate a simple concept based on in-situ Selective Laser Heat Treatment (SLHT) that can effectively alter the microstructure and activate the decomposition of the alpha '-martensite into a lamellar alpha +beta microstructure within a short time scale (similar to 30 s). SLHT consists of multiple rescanning of the printed part, with low energy density, triggering solid-state phase transformations. Operando X-ray diffraction has been performed on cuboid and thin wall geometries, and was augmented by thermal finite element simulations. Upon SLHT, a gradual formation of the beta phase as well as an alpha/ peak narrowing trend have been evidenced through X-ray diffraction, as an indication of the diffusional nature of alpha '-martensite decomposition. Moreover, through fine tuning of the process parameters at the final stage of SLHT, a controlled temperature evolution during cooling was achieved, leading to preservation of the beta phase, a product of the decomposition, down to room temperature. Complementary microstructural characterizations via EBSD, SEM, and TEM confirm the presence of a lamellar alpha +beta microstructure after SLHT. Our results evidence, for the first time, the fast kinetics of alpha '-martensite decomposition under in-situ SLHT. The approach is meant to be implemented at selected locations during the LPBF process, avoiding time-consuming post processing steps, and leading to composite-like, architected microstructures.