An object of the present invention is provide a method for sulfating a glycosaminoglycan in a solution of a non-organic solvent. In the present invention, sulfation reaction of a glycosaminoglycan is performed with a sulfating agent in a strongly basic solution of a non-organic solvent. In the present invention, pH of the strongly basic solution is preferably set to be 11.5 or higher. According to the present invention, for example, a glycosaminoglycan having heparin-like anticoagulant activity can be produced from N-acetylheparosan through one-pot procedure. In one embodiment, a sulfated glycosaminoglycan produced by the method of the present invention has a unique disaccharide composition and is expected to be a novel useful material.