Numerical simulations have been performed of a synthetic jet interacting with a laminar hypersonic boundary layer. Two datum cases were also considered, no jet and steady jet. The simulations for the case of no jet are in agreement with available experimental data. Predicted flow features of the steady jet interaction are broadly consistent with previous studies. For the synthetic jet, the upstream and downstream separated regions are dramatically reduced in size, and the jet appears to lie closer to the surface, compared with the steady jet. It is also found that the synthetic jet induces a greater mixing rate than the steady jet.