The interaction of a spatially transitional boundary layer flow at freestream Mach number M = 2.0 with an impinging oblique shock wave (β = 35.56) is studied by means of direct numerical simulation (DNS). High amplitude (5%) three-dimensional isotropic disturbances are superimposed on the laminar profile for a Reynolds number based on the boundary layer displacement thickness of Re_δ*0 = 1000 at the inlet plane of the computational box. Under the selected flow conditions, the interaction of the boundary layers with the incident shock wave produces three-dimensional separation, thus inducing a shock system (reflected waves and expansion fan). Linear-stability theory and DNS indicate that Λ-shaped vortices are generated in the separated shear layer due to the most unstable oblique mode, which can trigger the breakdown mechanism around the reattachment line. Energy spectral density of the wall-pressure indicates broadband spectra that are observed in the experimental result on shock/turbulent boundary layer interaction.