Some protein components of intracellular non-membrane-bound entities， such as RNA granules， are known to form hydrogels in?vitro. The physico-chemical properties and functional role of these intracellular hydrogels are difficult to study， primarily due to technical challenges in probing these materials in situ. Here， we present iPOLYMER， a strategy for a rapid induction of protein-based hydrogels inside living cells that explores the chemically inducible dimerization paradigm. Biochemical and biophysical characterizations aided by computational modelling show that the polymer network formed in the cytosol resembles a physiological hydrogel-like entity that acts as a size-dependent molecular sieve. We functionalize these polymers with RNA-binding motifs that sequester polyadenine-containing nucleotides to synthetically mimic RNA granules. These results show that iPOLYMER can be used to synthetically reconstitute the nucleation of biologically functional entities， including RNA granules in intact cells.