PROBLEM TO BE SOLVED: To provide a copper nanoparticle that is available by a convenient method without using a hydrogen-donating reductant, has a narrow particle size distribution, and can be dispersed in water.SOLUTION: A hydrophilic polymer-coated copper nanoparticle has a copper nanoparticle having an average particle size of 10-100 nm and a relative standard deviation of particle sizes of 30% or less, and a coating layer comprising a hydrophilic polymer disposed on the surface of the copper nanoparticle.SELECTED DRAWING: None