In an example, a composite stringer assembly includes a composite stringer, a radius filler, and an overwrap layer. The composite stringer includes: (i) a skin flange configured to be coupled to a support structure, (ii) a web, (iii) a lower corner portion of the composite stringer extending from the skin flange to the web, and (iv) an inner surface of the composite stringer extending along the skin flange, the lower corner portion, and the web. The radius filler includes a first surface coupled to the inner surface at the lower corner portion, a second surface configured to couple to the support structure, and a third surface extending between the first surface and the second surface. The overwrap layer is coupled to the inner surface at the web, the third surface of the radius filler, and the support structure.