A computational study has been carried out to assess the effectiveness of a porous medium as a passive control device suitable for reducing the drag in a normal-shock-wave/boundary-layer interaction at transonic speeds with a view toward application in aircraft wings. Reduction in overall drag is achieved via recirculation inside the porous medium, which primarily weakens the shock structure and hence reduces the wave drag. The study has been carried out for a Mach 1.3 normal-shock-wave/boundary-layer interaction on a flat plate in the presence of a porous medium beneath the region of interaction. The computations are performed as steady-state RANS calculations using Menter’s SST k- ω/ k- ϵ model for turbulence closure. A parametric study that investigates the dependency of the effectiveness of control on dimensions of the cavity (length and depth), relative position of the cavity, and porosity of the medium has been carried out. It is observed that the change in overall drag is pronounced for parameters which result in significant changes to the size of the lambda-shock structure, such as the length of the cavity upstream of the inviscid shock location. Among the parameters investigated, porosity is seen to strongly affect the boundary-layer properties, with increase in porosity resulting in higher viscous drag. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.