We report on the phase transformation of amorphous PtMnSb thin films induced by laser annealing in the nanosecond time regime. Structural and magnetic transformations are investigated by TEM, XRD, AFM and in-situ MOKE and VSM. We have established that a minimum laser fluence is required to crystallize the amorphous films and thus, to induce magnetic activity. The transformation kinetics vs number of irradiation pulses reveals that the magnetically active C1b phase is formed via an intermediate phase, namely, tetragonal-PtMn. We have also established that the thin film crystallization induced by the nanosecond laser annealing proceeds via nucleation rather than grain growth. Measurement of the lattice parameter of the C1b-PtMnSb produced by the laser quenching (LQ) indicates an essentially unstrained structure with a =6.17 Å vs a =6.201 Å reported for the bulk. Nevertheless, we observe the generation of large surface undulations upon laser annealing and suggest that this is the mechanism for stress relaxation concomitant with the large volumetric changes involved in the phase transformation. In addition, we observe decrements in saturation moments and Curie temperatures which are attributed to the nanocrystalline nature of the LQ specimens.