We report the observation of a narrow peak in the ${K}_{S}{K}^{\\ensuremath{-}}{\\ensuremath{\\pi}}^{+}$ invariant mass distribution in a sample of exclusive $B\\ensuremath{\\rightarrow}K{K}_{S}{K}^{\\ensuremath{-}}{\\ensuremath{\\pi}}^{+}$ decays collected with the Belle detector at the KEKB asymmetric ${e}^{+}{e}^{\\ensuremath{-}}$ collider. The measured mass of the peak is $M=3654\\ifmmode\\pm\\else\\textpm\\fi{}6(\\mathrm{s}\\mathrm{t}\\mathrm{a}\\mathrm{t})\\ifmmode\\pm\\else\\textpm\\fi{}8(\\mathrm{s}\\mathrm{y}\\mathrm{s}\\mathrm{t})\\text{ }\\text{ }\\mathrm{M}\\mathrm{e}\\mathrm{V}/{c}^{2}$, and we place a 90% confidence level upper limit on the width of $\\ensuremath{\\Gamma}l55\\text{ }\\text{ }\\mathrm{M}\\mathrm{e}\\mathrm{V}/{c}^{2}$. The properties agree with heavy-quark potential model expectations for the ${\\ensuremath{\\eta}}_{c}(2S)$ meson, the $n=2$ singlet $S$ charmonium state.
