An assembly for an aircraft and comprising a wing comprising an airfoil box partly produced using a front spar, a mounting pylon arranged under the wing and comprising a primary structure formed as a primary box having a top spar extending at least partly under the airfoil box and having a slit, a bottom spar, lateral panels and a transverse reinforcing fixing rib, of which a bottom part is housed and fixed inside the primary structure and of which a top part passes through the slit. A fixing arrangement fixes the top part to the front spar. The fixing arrangement comprises a plurality of fixing bolts, where each fixing bolt fixes the top part to the front spar and has its axis at right angles to the fixing rib. Such an assembly allows the mounting pylon primary structure to be as close as possible to the wing.