Abstract A well-resolved numerical simulation of a Mach 0.5 jet exiting from a rectangular shaped nozzle with an aspect ratio of 5 into a quiescent ambient was performed at a Reynolds number of 2000 based on the narrow side of the nozzle. The transition process was triggered by the most unstable symmetric linear eigenmode of a parallel laminar jet profile at the nozzle exit. We observe a shear-layer roll-up for the laminar jet and subsequently a very rapid transition to small-scale turbulence. The asymmetry of the velocity profile with respect to the jet axis leads to a rapid spreading of the jet along its minor axis within the so-called characteristic region as expected from experimental observations for similar configurations.