  Purely leptonic $B$ meson decays provide unique probes for physics Beyond the
Standard Model. We study the impact of a scalar leptoquark, $S_1$, on $B \to
\mu \bar\nu$ decay. We find that, for $m_{S_1}\sim 1$ TeV, the $S_1$ leptoquark
can modify the $B \to \mu \bar\nu$ rate significantly. Such a leptoquark can in
prinicple also alter the $B \to \tau \bar\nu$ rate. However, current searches
from LHC and low energy physics provide some constraints on the parameter
space.
