Abstract: The invention relates to a process for manufacturing stiffened structures (1) in composite materials formed by an outer skin (2) and a plurality of stiffeners (3) the cross-section of which has a closed shape delimiting an inner opening (6), comprising the following steps: a) providing a shaping jig (34); b) providing auxiliary male jigs (36) with inner cavities (32) reinforced (33) along their length, coated by membranes (37) suitable for curing; c) providing stiffeners (3); d) arranging the stiffeners (3) in the jig (34) and in their inner openings (6) the auxiliary male jigs (36); e) laminating the outer skin (2); f) curing the stiffened structure (1) with high temperature and pressure; g) removing the auxiliary male jigs (36) after reducing their section (41); h) separating the cured stiffened structure (1) from the jig (34), the design of the inner cavities (32) including their reinforcement (33) and the capacity to maintain or reduce their section by the application of overpressure or underpressure resp