The thermodynamic properties and electronic structure of hydrated Ra²⁺ have been investigated using ab initio quantum chemical calculations that apply the relativistic model core potential method and compared with those of the other hydrated divalent alkaline earth metal ions (Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺). The solvation free energies calculated for [Ra (H₂O)_n]²⁺ (n = 1–9) in a continuum dielectric media (semi-continuum model) showed that the hydration number of Ra²⁺ is in the range of 7–9. Natural population analysis (NPA), natural bond orbital (NBO) analysis and localized molecular orbital energy decomposition analysis (LMO-EDA) showed that the dominant interaction between hydrated Ra²⁺ ions and solvent water molecules is electrostatic interaction to form coordination bonds which have a strong ionic bond character. On the other hand, not only electrostatic interaction but also covalent interaction accompanying charge-transfer from solvent water molecules to the central ion are important in the interaction between hydrated Mg²⁺ or Ca²⁺ (lighter divalent alkaline earth metal ions) and solvent water molecules.