We report the synthesis and characteristic thermal properties of the adamantane-based ionic salts. The ionic salts with adamantane carboxylates and imidazolium cations were prepared via neutralization. It was found that the assembly of the ion pairs on the adamantane core can improve the thermal stability and lower melting temperatures even though the molecular weight increases. Finally, the thermally-stable room temperature-ionic liquids, which are defined as a salt with the melting temperature below 25 °C, can be obtained based on adamantane. The tetrahedral-like distribution of ion pairs could be responsible for simultaneously improving thermal stability and lowering melting temperatures of ion pairs.