The origin and role of S3 boundaries during dynamic recrystallization (DRX) and grain boundary engineering (GBE) of a Ti-modified austenitic stainless steel (alloy D9) is studied. Hot deformation tests were carried out on solution-annealed (SA) specimens to study the DRX behavior whereas a series of cold deformation and annealing were performed on SA specimens to realize GBE microstructure. A linear relationship between the area fraction of DRX and the number fraction of Σ3 boundaries was observed during hot deformation. This high fraction of Σ3 boundaries could account for the formation of coherent annealing twins by “growth accidents” during DRX. For certain combinations of cold deformation and annealing, a significant increase in S3 boundaries was observed. In contrast to hot deformation, majority of these new S3 boundaries during cold deformation and annealing were formed by geometrical interactions between the pre-existing Σ3 boundaries. The role of the S3 boundaries during DRX and on tailoring microstructure through grain boundary engineering approach is discussed.