<p>A highly concentrated borate solution stable at room temperature is successfully prepared by mixing methylamine and boric acid. The boron concentrations of saturated solutions are strongly dependent on B/N molar ratios, and the highest concentration, 33.4&#xa0;mol⋅kg⁻¹ as to boron, is achieved at a B/N ratio of 1.55 at 25&#xa0;°C. A new stable methylammonium borate, whose chemical formula is estimated as CH₃NH₃B₅O₈⋅6H₂O, is precipitated by drying the solution at a B/N ratio of 5.0. The saturation concentration of the salt at the B/N ratio of 5.0 is 1.94&#xa0;mol⋅kg⁻¹, much lower than that of the solution at the B/N ratio of 1.55. Interestingly, the highly concentrated solutions at the B/N ratio less than 5.0 form the same solid salt as that obtained at the B/N ratio of 5.0 by drying. The Raman spectra of the solutions suggest that the structures of borate ions change significantly depending on the B/N ratios, which can be related to the high solubility. The methylammonium borate solutions are expected to be widely used as new, highly concentrated ones containing much boron for industrial applications, such as flame retardants, termiticides, and neutron absorbers.</p>