The present invention is intended to offer a novel material which combines cell-adhesive proteins such as collagen with GAG polymers to construct an environment similar to an extracellular matrix, capable of controlling differentiation and proliferation of cells. The present invention offers a glycosaminoglycan-functionalized polymer protein composite produced by combining a cell-adhesive protein with a glycosaminoglycan-functionalized polymer obtained by incorporating a carbohydrate chain containing a structure corresponding to at least a portion of a glycosaminoglycan backbone into a vinyl polymer main chain, as well as a cell culture substrate and a material for tissue reconstruction treatments comprising the composite.