One and two scandium ions (Sc3+) are bound strongly to nonheme manganese(IV)–oxo complexes, (N4Py)MnIV(O)2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) and (Bn-TPEN)MnIV(O)2+ (Bn-TPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)-1,2-di­amino­ethane), to form MnIV(O)–(Sc3+)1 and MnIV(O)–(Sc3+)2 complexes, respectively. The binding of Sc3+ ions to the MnIV(O) complexes was examined by spectroscopic methods as well as by DFT calculations. The one-electron reduction potentials of the MnIV(O) complexes were markedly shifted to a positive direction by binding of Sc3+ ions. Accordingly, rates of the electron transfer reactions of the MnIV(O) complexes were enhanced as much as 107–fold by binding of two Sc3+ ions. The driving force dependence of electron transfer from various electron donors to the MnIV(O) and MnIV(O)–(Sc3+)2 complexes was examined and analyzed in light of the Marcus theory of electron transfer to determine the reorganization energies of electron transfer. The smaller reorganization energies and much more positive reduction potentials of the MnIV(O)–(Sc3+)2 complexes resulted in remarkable enhancement of the electron-transfer reactivity of the MnIV(O) complexes. Such a dramatic enhancement of the electron-transfer reactivity of the MnIV(O) complexes by binding of Sc3+ ions resulted in the change of mechanism in the sulfoxidation of thioanisoles by MnIV(O) complexes from a direct oxygen atom transfer pathway without metal ion binding to an electron-transfer pathway with binding of Sc3+ ions.