The surface stiffness of a fluid‐saturated porous solid is defined as the ratio between a small change in capillary pressure and the average displacement of the boundary due to the resulting rise or fall of the fluid level in the pore channels. When the surface pores are structurally open, the surface stiffness is entirely due to the stiffness of the microscopic fluid membranes extended by capillary forces over the surface pores. Due to interfacial tension between the immiscible wetting fluid in the pores and nonwetting fluid (air) above the surface, essentially closed‐pore boundary conditions can prevail at the interface. It has recently been shown that the surface stiffness of a porous material containing cylindrical pores can be calculated simply as the surface tension of the saturating fluid divided by the static permeability of the porous solid [P. B. Nagy, Appl. Phys. Lett. 60, 2735 (1992)]. In this letter, we show that the same simple relationship can be generalized for the surface stiffness of fluid‐saturated porous media containing parallel prismatic pore channels of any number, size, or shape. © 1995 American Institute of Physics.