An internal mammary prosthesis, implantable inside the body of the patient in reconstructive surgery, has a solid body of polymer foam material having a density between 0.10 g/cm 3 and 0.50 g/cm 3 so as to be particularly lightweight, the body being provided with a coating and comprising a front portion and a back, and having defined, on a cross-section of the prosthesis in a transverse plane virtually sectioning the sternum of a patient when the prosthesis is in use: - the sternal point, at the most medial point of the prosthesis, and the lateral thoracic point, at the most lateral point of the prosthesis, the sternal point and the lateral thoracic point delimiting the base of the prosthesis; - a plane α passing through the sternal point and parallel to the sternum; - a plane β passing through the lateral thoracic point and parallel to the sternum; - the height as the distance between the planes α and β; - a plane Ω passing through the lateral thoracic point and orthogonal to the sternum; - the point of intersection between the planes α and Ω; - the length as the distance between the sternal point and the point of intersection; - the points L 1/3, at a distance equal to one third of the length calculated starting from the sternal point, and L 2/3, at a distance equal to two thirds of the length calculated starting from the sternal point; - along the plane Ω, the points H 1/3 at a height equal to one third of the height calculated starting from the point of intersection, and H 2/3, at a height equal to two thirds of the height calculated starting from the point of intersection; - the plane Δ passing through the point L 2/3 and orthogonal to the plane α; - the plane μ passing through the point H 1/3 and orthogonal to the plane Ω; - the reference points P 1/3 as corresponding to L 1/3, and P 2/3 as given by the intersection between the planes Δ and μ; the internal mammary prosthesis being characterized in that the the back of the body has, in cross-section, a curvature obtained by interpolating at least the sternal point, the reference point P 2/3 and the lateral thoracic point. - the sternal point, at the most medial point of the prosthesis, and the lateral thoracic point, at the most lateral point of the prosthesis, the sternal point and the lateral thoracic point delimiting the base of the prosthesis; - a plane α passing through the sternal point and parallel to the sternum; - a plane β passing through the lateral thoracic point and parallel to the sternum; - the height as the distance between the planes α and β; - a plane Ω passing through the lateral thoracic point and orthogonal to the sternum; - the point of intersection between the planes α and Ω; - the length as the distance between the sternal point and the point of intersection; - the points L 1/3, at a distance equal to one third of the length calculated starting from the sternal point, and L 2/3, at a distance equal to two thirds of the length calculated starting from the sternal point; - along the plane Ω, the points H 1/3 at a height equal to one third of the height calculated starting from the point of intersection, and H 2/3, at a height equal to two thirds of the height calculated starting from the point of intersection; - the plane Δ passing through the point L 2/3 and orthogonal to the plane α; - the plane μ passing through the point H 1/3 and orthogonal to the plane Ω; - the reference points P 1/3 as corresponding to L 1/3, and P 2/3 as given by the intersection between the planes Δ and μ;