The defect structure resulting from copper doping of potassium niobate (KNbO3) ceramics was investigated by a combined analysis of electron paramagnetic resonance (EPR) spectroscopy and first-principles calculations based on density functional theory (DFT). The results indicate that under atmospheric oxygen partial pressure, Cu preferentially substitutes on the Nb sites where it can trap one or two oxygen vacancies. Correspondingly, for 0.25 mol% Cu doped KNbO3 ceramics, two types of defect associates are formed - (Cu'''(Nb) - V-O(center dot center dot))' and (V-O(center dot center dot) - Cu'''(Nb) - V-O(center dot center dot))(center dot). The association of Cu impurities and oxygen vacancies lowers the defect formation energy by 1.0eV to 2.7 eV, depending on the Fermi level. Owing to the opposite charges of these two types of defects, overall charge neutrality is theoretically possible by mutual compensation of the defect associates, without formation of additional non-associated oxygen vacancies.