  We report the observation of a narrow peak in the KsK-pi+ invariant mass
distribution in a sample of exclusive B-->K KsK-pi+ decays collected with the
Belle detector at the KEKB asymmetric energy e+e- collider. The measured mass
of the peak is M=3654+-6(stat)+-8(syst) MeV/c^2 and we place a 90% confidence
level upper limit on the width of Gamma<55 MeV/c^2. The properties agree with
heavy-quark potential model expectations for the eta_c(2S), the n=2 singlet S
charmonium state.
