Junction flows occur when a boundary layer develops on a wall and encounters an obstacle protruding from this surface. When the obstacle generates enough of an adverse pressure gradient to separate the flow, the aerodynamic drag is increased. In this paper, aerodynamic shape optimization (ASO) is employed to optimize a wing/body junction geometry at a chordReynolds number ofReC = 9.7 105,where thewing is theprotrusionandthebodyis representedby a flat plate. In contrast to conventional ASOs, thewing shape is kept fixed and only deformations of the body are allowed in order to study its influence on the junction drag. The obtained optimized design is characterized by a concave shape similar to a dent in the junction area and differentiates itself from the traditional convex fairings. For this reason, it is named the anti-fairing. Wind-tunnel experiments using stereoscopic particle image velocimetry in the wake of the junction area and a new set of Reynolds-averaged Navier-Stokes simulations with a finermesh than that used in the optimization are performed in order to validate the optimization, estimate the drag reduction with respect to the baseline geometry and two different leading-edge fairings, and investigate the mechanism by which drag is reduced. The anti-fairing is shown to systematically reduce drag and outperform leading-edge fairings thanks to the interaction between the wing and the front part of the concavity, generating a pressure force in the direction opposite to the drag force.