A crosslinkable polysaccharide derivative which has in the side chain of polysaccharide at least one active ester group reactive with an active hydrogen-containing group and which forms a crosslinked product through covalent bond between the active ester group and the active hydrogen-containing group upon contact with alkaline water. A composition and a medical treatment material containing the crosslinkable polysaccharide derivative. The crosslinkable polysaccharide derivative produces a high bond strength that meets clinical requirements. It avoids the risk of infection because it is based on a material which is not derived from living organisms. It in itself and its decomposition products have a low level of toxicity because it is formed from a synthetic material. It is biodegradable and bioabsorbable. It can be prepared readily and simply without requiring special apparatus at the time of use.