  We present a new mechanism for deep crustal heating in accreting neutron
stars. Charged pions ($\pi^+$) are produced in nuclear collisions on the
neutron star surface during active accretion and upon decay they provide a flux
of neutrinos into the neutron star crust. For massive and/or compact neutron
stars, neutrinos deposit $\approx 1\textrm{--} 2 \, \mathrm{MeV}$ of heat per
accreted nucleon into the inner crust. The strength of neutrino heating is
comparable to the previously known sources of deep crustal heating, such as
from pycnonuclear fusion reactions, and is relevant for studies of cooling
neutron stars. We model the thermal evolution of a transient neutron star in a
low-mass X-ray binary, and in the particular case of the neutron star
MXB~1659-29 we show that additional deep crustal heating requires a higher
thermal conductivity for the neutron star inner crust. A better knowledge of
pion production cross sections near threshold would improve the accuracy of our
predictions.
