We calculate the full electroweak one-loop corrections to the decay of the CP-odd Higgs boson ${A}^{0}$ into scalar fermions in the minimal supersymmetric extension of the standard model (MSSM). For this purpose many parameters of the MSSM have to be properly renormalized in the on-shell renormalization scheme. We have also included the supersymmetric QCD corrections. For the decay into bottom squarks and tau sleptons, especially for large $\mathrm{tan}\ensuremath{\beta},$ the corrections can be very large, making the perturbation expansion unreliable. We solve this problem by an appropriate definition of the tree-level coupling in terms of running fermion masses and running trilinear couplings ${A}_{f}.$ We also discuss the decay of heavy scalar fermions into light scalar fermions and ${A}^{0}.$ We find that the corrections can be sizable and therefore cannot be neglected.