However, the hydrophobicity of conventional MD membranes makes them vulnerable to fouling when treating saline water containing organic compounds. [5] Taking the produced water as an example, total dissolved organic carbon up to 5 g L −1 has been reported. [6] The organic contaminants not only block membrane pores, but also lead to membrane wetting by the low-surface-tension compounds, resulting in the reduction of both permeate flux and desalination efficiency during MD operations. [5b] To alleviate the fouling problem, surfaces with ultralow surface energy were constructed to repel both water and lowsurface-tension compounds. [2b,7] The omniphobicity prevented membrane from oil fouling when treating feed solution with 10 mg L −1 oil. [8] However, the permeate flux of the omniphobic membranes declined as oil concentration increased to 80 mg L −1 , [9] indicating the limitation in improving MD fouling resistance by merely reducing membrane surface energy.Membranes with the combination of hydrophilic surface and hydrophobic structure present the advantage of repelling oil foulants while maintaining the wetting resistance. [10] Such Janus membranes have been directly fabricated by forming layers with opposite wettability via sequential electrospinning [11] or postcasting phase separation, [12] but materials available for direct asymmetric fabrication were quite limited. Asymmetric modification of the hydrophobic MD membranes has been studied as an alternative for the preparation of Janus membranes. A variety of hydrophilic polymers, including chitosan, [13] polydopamine, [14] polyvinyl alcohol, [15] and zwitterionic polymers, [16] were selectively processed onto MD membranes using solvent-based methods such as spray-and dip-coating. Chemical reaction in alkaline solution has also been reported to introduce hydrophilicity to the MD membrane surface. [17] Despite these advances, several challenges remained in the asymmetric modification of MD membranes. The hydrophilic modification usually required polymer preparation and/or processing in various solvents for long periods of time, [14,16] and post-treatments were often needed to remove the solvent residue and cure the coatings. [11][12][13][14] In addition, the conflicting wettability of hydrophilic and hydrophobic components made the solvent-based methods less effective and resulted in poor interfacial compatibility. [18] To facilitate the attaching of hydrophilic coatings onto the liquid-repelling hydrophobic structure, protocols such as plasma activation, [16] chemical modification, [19] Janus membranes with excellent fouling resistance and mass transport properties are highly desired to improve the performance and efficiency in the treatment of oil-contaminated wastewater. The fabrication of Janus membranes requires control of both the membrane microstructure and the interfacial compatibility between distinct components. This work presents a vapor-based strategy to incorporate Janus property into porous substrates without the wetting incompatibility in solut...