Formulas are developed which define the plastic buckling coefficients for a simply supported plate stiffened by longitudinal stiffeners and subject to uniform compression in the direction of the stiffeners. The equations are developed by consideration of the strain energy of plastic buckling. The cases considered in detail are the plate stiffened by one, two, and an infinite number of stiffeners. I t is shown that design charts of the buckling coefficients can be readily developed for very long stiffened plates. An approximate method is presented which permits evaluation of the plastic buckling coefficient for stiffened plates with any aspect ratio.