The surface wettability of porous carbon substrates is a critical factor dictating the performance of electrochemical devices such as fuel cells and supercapacitors. In this study, a single-step oxygen plasma treatment technique is employed to create a wettability gradient on the fibrous carbon substrate in the direction perpendicular to the layer's plane. The wetting properties of the as-received and plasma-treated carbon layers are investigated via coupled optical and electrochemical techniques. Surface characterization revealed that the plasma treatment is effective in creating oxygen-containing functional groups throughout the bulk of the fibrous carbon layer while preserving the substrate morphological integrity. The plasma modification penetrated the porous carbon network, generating a through-plane gradient in the functional groups and wetting properties. Plasma treatment on the microporous layer (MPL) side of the porous transport layer (PTL) in a proton exchange membrane fuel cell (PEMFC) enabled improved performance, compared to the same treatment on the gas diffusion layer (GDL) side. We explain the discrepancy in the effectiveness of the GDL-treated and MPL-treated surfaces in regulating water transport based on the difference in the surface wettability induced by the pore sizedependent plasma penetration depth.