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Magnetoelectric coefficient values of above 5 and 2 V cm–1 Oe–1 in 20 nm CoFe2O4–BaTiO3 and NiFe2O4–BaTiO3 core–shell magnetoelectric nanoparticles were demonstrated. These colossal values, compared to 0.1 V cm–1 Oe–1 commonly reported for the 0–3 system, are attributed to (i) the heterostructural lattice-matched interface between the magnetostrictive core and the piezoelectric shell, confirmed through transmission electron microscopy, and (ii) in situ scanning tunneling microscopy nanoprobe-based ME characterization. The nanoprobe technique allows measurements of the ME effect at a single-nanoparticle level which avoids the charge leakage problem of traditional powder form measurements. The difference in the frequency dependence of the ME value between the two material systems is owed to the Ni-ferrite cores becoming superparamagnetic in the near-dc frequency range. The availability of novel nanostructures with colossal ME values promises to unlock many new applications ranging from energy-efficient information processing to nanomedicine and brain–machine interfaces.