A shim adapted for positioning on the upper surface of at least one wing of an aircraft. The shim restores an aerodynamic profile of the wing at the position of a zone of transition between structure fastening the wing to the fuselage of the aircraft and the upper surface of the aircraft. The shim enables a seal to run from the fastening structure to the upper surface of the wing, in eliminating the zones at which the seal may break open. An aircraft provided with such a shim is also disclosed.