An expansion folder with internal dividers with an accordion-pleated spline, comprising, a front cover, the front cover comprising an outwardly facing surface and an inwardly facing surface, a rear cover, the rear cover comprising an outwardly facing surface and an inwardly facing surface, a plurality of dividers, each of the plurality of dividers comprising a front-facing surface and a rear-facing surface, and, an accordion-pleated spline, the spline comprising a polymer formed into a plurality of pleats, the pleats further comprising an alternating series of “M-shaped” and “W-shaped” pleats.