A process for manufacturing stiffened structures (11) in composite materials formed by an outer coating (13) and a plurality of stiffeners (15) the cross-section of which has a closed form delimiting an inner opening (23), comprising the following steps: a) Providing a shaping tool (31); b) Providing auxiliary male tools (37) with an inert material inside them coated with membranes suitable for curing c) Providing stiffeners (15) in fresh or cured condition; d) Arranging the stiffeners (15) in the tool (31) and the auxiliary male tools (37) in their inner openings (23); e) Laminating the outer coating (13); f) Curing the stiffened structure (11) with high temperature and pressure; g) Removing the auxiliary male tools (37) after reducing their volume by withdrawing the inert material thereof; h) Separating the cured stiffened structure (11) from the tool (31). The invention also relates to the auxiliary male tools.