We discovered that an amorphous-crystalline transition occurs when a small amount of water is introduced into an amphiphilic copolymer network consisting of n-alkyl acrylate (n = 14, 16, 18, 20, 22) and acrylic acid. Differential scanning calorimetry and X-ray diffraction studies show that the dry polymer network is in an amorphous state regardless of the presence of the crystalline moiety. By introduction of water to the polymer network, a crystalline structure attributed to the side-by-side packing of the alkyl chain is formed and the melting temperature of the crystalline increases with the amount of water and saturated to that of homo-n-alkyl acrylate polymers at a water content of 5 wt %. The mechanism of the water-induced amorphous-crystalline transition is explained in terms of enhanced mobility of the main chain due to hydration of the carboxyl groups of the polymer network.