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The existence of an intermediate phase of tin oxide was first reported in 1882. However, its stoichiometry and its crystal structure have been dubious and heavily debated ever since, despite a multitude of structural investigations. We show that Raman spectroscopy combined with ab initio theory offers a viable alternative for structure determination in cases where diffraction studies are inconclusive. We unambiguously identify the intermediate phase as ${\mathrm{Sn}}_{3}{\mathrm{O}}_{4}$ and rule out the other likely candidate, ${\mathrm{Sn}}_{2}{\mathrm{O}}_{3}$. We assign the one-phonon Raman signals of ${\mathrm{Sn}}_{3}{\mathrm{O}}_{4}$ to the mode symmetries of the corresponding point group ${C}_{2h}$ and confirm that $P{2}_{1}/c$ with 14 atoms per unit cell is the space group of ${\mathrm{Sn}}_{3}{\mathrm{O}}_{4}$.