We investigate the electronic structure and the ferroelectric properties of the recently discovered multiferroic ScFeO3 by means of ab initio calculations. The 3d manifold of Fe in the half-filled configuration naturally favors an antiferromagnetic ordering, with a theoretical estimate of the antiferromagnetic Ne ́el temperature in good agreement with the experimental values. We find that the inversion symmetry breaking is driven by the off-centering of Sc atoms, which results in a large ferroelectric polarization of ∼105 μC/cm2. Surprisingly the ferroelectric polarization is sensitive to the local magnetization of the Fe atoms resulting in a large negative magnetoelectric interaction. This behavior is unexpected in type-I multiferroic materials because the magnetic and ferroelectric orders are of different origins.