The Coulomb blockade across Fe and Pd nanoislands grown on single-crystalline MgO insulator films on Fe(001) was investigated with scanning tunneling microscopy. In I(U) spectra of individual islands, the characteristic steps of the Coulomb staircase were observed as a function of island size. A detailed analysis of the Coulomb staircase energetics in terms of the island capacitane reveals the expected linear behavior for electrostatic interactions. For ferromagnetic Fe islands, however, a significant offset of 1 eV was found, which is missing in case of nonmagnetic Pd islands. This effect is explained by the spin dependence of the electronic transport across the MgO barrier in combination with a quasihalf-metallic density of states of the Fe island, as corroborated by first-principles electronic-structure calculations.