In this work, hindered rotations of the BH₄ ¯ tetrahedra in Mg(BH₄)₂ were studied by quasielastic neutron scattering, using two instruments with different energy resolution, in combination with density functional theory (DFT) calculations. Two thermally activated reorientations of the BH₄ ¯ units, around the 2-fold (C₂) and 3-fold (C₃) axes were observed at temperatures from 120 to 440 K. The experimentally obtained activation energies (E_aC2 = 39 and 76 meV and E_aC3 = 214 meV) and mean residence times between reorientational jumps are comparable with the energy barriers obtained from DFT calculations. A linear dependency of the energy barriers for rotations around the C₂ axis parallel to the Mg-Mg axis with the distance between these two axes was revealed by the DFT calculations. At the lowest temperature (120 K) only 15% of the BH₄ ¯ units undergo rotational motion and from comparison with DFT results it is expectedly the BH₄ ¯ units with the boron atom closest to the Mg-Mg axis, although dynamics related to local disorder existing at the boundary of the antiphase domains or to the presence of solvent in the sample cannot be strictly excluded. No long-range diffusion events were observed.