Polymeric membranes for water purification are faced with fouling by the aggregation of contaminants on the surface, decreasing water permeance. Herein, we demonstrate facile fluorination of ultrafiltration (UF) membranes by directly coating a "nonstick" amorphous perfluoropolymer of Teflon AF1600, achieving optimally low surface energy with excellent antifouling properties, while retaining surface pore structures and thus high water permeance. At the Teflon AF1600 concentration of 200 ppm, the derived fluorinated layer has a thickness less than 5 nm and shows an unexpected surface energy of 24 mN/m, very close to that with the minimal biological fouling on the Baier's curve. When challenged with 0.5 g/L bovine serum albumin solution in a constant-flux crossflow system, the fluorinated polyacrylonitrile (PAN) UF membrane showed a 54% lower fouling rate than the pristine one at a water flux of 60 L/(m(2) h). This approach enables flexible control of the degree of fluorination and surface energy and can be easily implemented in current membrane manufacturing processes.