The interface magnetization and its modulation is explored in oxide heterostructures within the framework of density functional theory. In particular, modulation in interface magnetization is investigated in prototype heterostructure SrRuO3|KNbO3(0 0 1) consisting ferromagnetic SrRuO3 and nonmagnetic KNbO3 as the thickness of KNbO3 is varied. The computed interfacial magnetization is found to alternate between a higher and a lower value with increasing KNbO3 thickness in the heterostructure. The average change in interface magnetization is computed to be ∼−60% as KNbO3 film changes from non-stoichiometric to stoichiometric and ∼+60% as it changes from stoichiometric to non-stoichiometric due to variation in thickness. The magnetic moment per interface cell changes from ∼2.27 μB for non-stoichiometric KNbO3 to ∼1.31 μB for stoichiometric KNbO3 in the heterostructure. The modulation in interface magnetization arises due to spin-dependent screening of effective charge of KNbO3 unit cells by accumulated free carriers at the interface. Large modulation in interface magnetization is also obtained for other heterostructures consisting magnetic substructure and ionized planes at the interface. In addition, the alteration in interface magnetization due to ferroelectric polarization reversal in ferromagnetic|ferroelectric SrRuO3|KNbO3 (0 0 1) superlattice is explored. The resulting interface magnetoelectric effect as well as control of interface magnetization using ionized planes may have interesting implications for technological applications. © 2018 Elsevier B.V.