Copper nanoparticles (CuNPs) are widely used and inevitably released into aqueous environments, causing ecological and health risks. Ubiquitous natural organic matter (NOM) might affect the copper release behaviors from CuNPs and their toxicity. This work aims to elucidate how NOM affects copper release from CuNPs, with a focus on the impacts of NOM properties and the NOM-CuNPs interaction mechanism. The copper release kinetics and different copper fractions induced by representative NOMs were characterized. The presence of NOM led to a more dispersive state of CuNPs clusters. Copper release mainly resulted from complexation reactions between CuNPs and functional groups of NOM. Humic substances were more effective in releasing copper than sodium alginate and bovine serum albumin, due to a higher amount of functional groups and lower molecular weight, which facilitated the contact and complexion reactions. Chlorination treatment of NOM significantly decelerated copper release due to the destruction of functional groups and less attachment of NOM. However, the copper releasing ability of humic acid was not substantially affected by Ca²⁺-induced coagulation. This study provides better understanding about the persistence and transformation of CuNPs in aquatic environments.