An object of the present invention is to provide a copolymer that is excellent in water permeability, suppression of platelet adhesion, and suppression of protein adhesion, and a separation membrane, a medical device, and a separation membrane module for medical use using the copolymer. The present invention provides a copolymer including monomer units derived from two or more types of monomers, wherein the hydration energy density of the copolymer is 158.992 to 209.200 kJ.mol·nm, the monomer unit with the highest hydration energy density in the monomer units is a monomer unit not containing a hydroxy group, the volume fraction of the monomer unit with the highest hydration energy density in the monomer units is 35 to 90%, and the difference in hydration energy density is 71.128 to 418.400 kJ·mol·nm.
