The molecular structure of magnesium dibromide was investigated by high-level computational techniques and gas-phase electron diffraction. The vapor consisted of about 88% monomeric and 12% dimeric species at the electron diffraction experimental conditions at 1065 K. The geometrical parameters and vibrational characteristics of monomeric, dimeric, and trimeric magnesium dibromide species were determined by computations. Very high level computations with extended basis sets and relativistic pseudopotentials on bromine were needed to reach an agreement between computed and estimated experimental equilibrium geometries for the monomer. For both the dimer and the trimer, different geometrical arrangements were tested. Their ground-state structures have halogen bridges with four-membered ring geometries and D-2h and D-2d symmetry, respectively. Thermodynamic parameters have also been calculated.