Example embodiments relate to a membrane, a method of manufacturing the same, and a composite membrane including the same. The membrane may include a polyacrylonitrile-based copolymer that includes a hydrophobic side chain and/or a hydrophobic repeating unit. The membrane may include a skin layer and a porous layer. A thickness ratio of the skin layer relative to the porous layer may be about 0.01 or less. The skin layer may have a thickness of about 1 μm or less. The membrane may have a relatively high water flux. When using the membrane, a water treatment module having higher energy efficiency may be achieved.