  We study the four-body semileptonic baryonic decay of $B^-\to \Lambda\bar
p\nu\bar \nu$ in the standard model. We find that the decay branching ratio is
$(7.9\pm 1.9)\times 10^{-7}$. Similar to the rare decays of $B^-\to
K^{(*)-}\nu\bar \nu$, this baryonic decay of $B^-\to \Lambda\bar p\nu\bar \nu$
is also sensitive to new physics and accessible to the future $B$ factories.
