Hot cracking during laser welding of Transformation Induce Plasticity (TRIP) steel at the edges of steel flanges can be a problem. In this study, modified hot cracking tests were performed by welding on a single-side clamped specimen at various distances from the free edge, while the heat input and external constraints remained constant. In situ temperature and strain measurements were carried out using pre-attached thermocouples and digital image correlation, respectively. A thermal-mechanical finite element (FE) model was constructed and validated with the temporal and spatial data measured. From the validated FE model, the temperature and strain evolution in the weld mushy zone were studied. A critical strain for the onset of hot cracking in the TRIP steel examined was found to be in the range of 3.2 to 3.6%. This threshold was further evaluated and experimentally confirmed by welding with different heat inputs. © 2017, The Author(s).