A method for manufacturing a composite-material casing for a gas turbine, includes producing by three-dimensional weaving a fiber texture in the form of a strip, winding of the fiber texture around several superimposed turns on a mandrel with a profile corresponding to that of the casing to be manufactured in order to obtain a fiber preform of a shape corresponding to that of the casing to be manufactured, and densifying the fiber preform by a matrix. During the winding of the last turn of the fiber texture on the mandrel, at least one stiffening element is interposed between the before-last turn and the last turn of the fiber texture. The stiffening element projects over the outer surface of the before-last turn of the fiber texture. The stiffening element has an axial section of omega-type shape.