  We present the first observation of the decay B->J/psi phi K. Using 9.6
million B meson pairs collected with the CLEO detector, we have observed 10
fully reconstructed B->J/psi phi K candidates, whereas the estimated background
is 0.5+/-0.2 events. We obtain a branching fraction of Br(B->J/psi phi K) =
(8.8+3.5/-3.0(stat)+/-1.3(syst))*10^-5. This is the first observed $B$ meson
decay requiring the creation of an additional s sbar quark pair.
