A device for manufacturing a stiffened panel with reinforcements which each have a final thickness and a final height. The manufacturing device includes tools configured to each support a preform of fibers having a U-shaped or C-shaped cross section with a base and two wings. Each tool comprises, at at least one of its first and second lateral faces, a set-back portion which, when two tools are juxtaposed, delimits a recess configured to form a reinforcement during a consolidation or polymerization step. The tools are dimensioned such that each recess has a width, at the consolidation or polymerization temperature, that is substantially equal to the thickness of the reinforcements. This solution makes it possible to more effectively control the geometry of the stiffened panel by avoiding the formation of beads. A method for manufacturing a stiffened panel using the device is also disclosed.