  We report the observation of the decay mode $B^{\pm}\to p \bar{p} K^{\pm}$
based on an analysis of 29.4 fb$^{-1}$ of data collected by the Belle detector
at KEKB. This is the first example of a $b\to s$ transition with baryons in the
final state. The $p \bar{p}$ mass spectrum in this decay is inconsistent with
phase space and is peaked at low mass. The branching fraction for this decay is
measured to be ${\cal B}(B^{\pm}\to p \bar{p} K^{\pm})
  =(4.3^{+1.1}_{-0.9}({\rm stat})\pm 0.5({\rm syst}))\times 10^{-6}.$ We also
report upper limits for the decays $B^0\to p \bar{p} K_S$ and $B^{\pm}\to p
\bar{p} \pi^{\pm}$.
