A reinforcement structure for an opening in the primary structure of an aircraft, the structure comprising a skin, frame members which are transverse with respect to the flying direction of the aircraft, and stringers which are longitudinal with respect to the flying direction of the aircraft. The reinforcing structure includes a perimetral reinforcing element situated along the edge of the opening and reproducing the geometrical form thereof; at least one pair of transverse reinforcing elements arranged on both transverse sides of the opening; and at least one pair of longitudinal reinforcing elements arranged on both longitudinal sides of the opening.