Natural convection heat transfer in a rectangular cavity filled with a saturated porous medium with variable permeability is investigated analytically and numerically. The side walls of the cavity are heated and cooled by constant heat fluxes, while the horizontal walls are adiabatic. The governing parameters for the problem are thermal Rayleigh number, R, the aspect ratio of the cavity, A, and the variable permeability constant, c. On the basis of the Darcy-Oberbeck-Boussinesq equations, a new analytical solution is developed in the limit of a long shallow cavity. A numerical study of the same phenomenon, obtained by solving the complete system of governing equations, is conducted to assess the validity of the analytical results. It is demonstrated that, for a wide range of the governing parameters, the flow structure and the heat transfer were well predicted by the analytical solution.