Water retention is a pervasive issue in agriculture and industry. Inspired by the water-storage mechanisms in plant cells, three kinds of polymeric microcapsules (PMCs) with carboxylic acid, sulfonic acid, and pyridyl groups are prepared using distillation-precipitation polymerization. The size of the lumen of the PMCs may govern the static water uptake by holding water molecules in a free-water state, and the functional groups in the shell of PMCs may manipulate dynamic water release by holding water molecules in a bound-water state, thus yielding PMCs with high and tunable water-retention properties. Incorporation of PMCs into composite membranes gives rise to dramatically enhanced water-retention properties and proton-transfer pathway, and consequently increased proton conductivity by up to one order of magnitude over the control polymer membrane, under low relative humidity of 20%. This study may offer a facile and generic strategy to design and prepare a variety of materials with superior water-retention properties.