The purpose of the present invention is to provide a copolymer having exceptional water permeability, platelet adherence suppression, and protein adherence suppression, and to provide a separation membrane, a medical device, and a medical separation membrane module in which said copolymer is used. The present invention provides a copolymer containing monomer units derived from two or more types of monomer, wherein the hydration energy density of the copolymer is 158.992 to 209.200 kJ·mol?1·nm?3, the monomer unit having the highest hydration energy density among the monomer units does not include a hydroxy group, the volume fraction of monomer units having the highest hydration energy density among the monomer units is 35 to 90%, and the difference in hydration energy density is 71.128 to 418.400 kJ·mol?1·nm?3.
