Here we demonstrate a novel technique for the fabrication of porous polymer membranes via vapor phase polymerization. Vapor phase processing allows for control over the chemical functionality of the membranes and eliminates solubility requirements and surface tension effects. Porous polymer membranes are formed by concurrent deposition of solid monomer and polymerization, which is achieved by increasing the partial pressure of the monomer above its saturation pressure and decreasing the substrate temperature below the freezing point of the monomer. The membranes exhibit dual-scale porosity, where the large-scale pores form during the deposition and the small-scale pores form upon sublimation of the solid monomer. We demonstrate that the growth rate and pore size of the membrane can be controlled by varying the reactor parameters, including deposition time, monomer partial pressure, and substrate temperature. Stimuli-responsive poly(methacrylic acid) and poly(N-isopropylacrylamide) membranes were fabricated to show the generality of the process. Furthermore, the ability to make copolymer membranes was demonstrated using ethylene glycol diacrylate as a cross-linking agent. Our ability to produce tailored polymer membranes with chemically diverse compositions has potential applications in separations and biosensing.