The microstructure and mechanical properties of dissimilar butt-joints between a highstrength low alloyed (HSLA) grade and an austenitic high Mn TRIP steel were investigated. The tool rotation and the tool offset toward the TRIP steel were varied between 300-500 rpm and 1-2 mm, respectively. Tool advancing speed amounted to 100 mm/min. Maximum tension stress was observed for the butt-joint welded with 300 rpm and 2 mm offset. The lowest increase in hardness within the stirred zone also occurred for this FSW condition, indicating that this tool rotation is more promising for welding dissimilar joints of commercial HSLA and high Mn TRIP steels. The weld microstructure consisted mainly of a stirred zone, and neither significant HAZ nor TMAZ are observed. However, two main lobular regions are observed, one at the bottom and another one at the top side of the welds. Besides, the HSLA develops a multiphase microstructure consisting of bainite, martensite and retained austenite phases, whereas no ε/α martensite is found in the stirred zone of the austenitic high-Mn TRIP steel.