  A consistent set of asymptotic conditions for the simplest supergravity
theory without cosmological constant in three dimensions is proposed. The
canonical generators associated to the asymptotic symmetries are shown to span
a supersymmetric extension of the BMS$_3$ algebra with an appropriate central
charge. The energy is manifestly bounded from below with the ground state given
by the null orbifold or Minkowski spacetime for periodic, respectively
antiperiodic boundary conditions on the gravitino. These results are related to
the corresponding ones in AdS$_3$ supergravity by a suitable flat limit. The
analysis is generalized to the case of minimal flat supergravity with
additional parity odd terms for which the Poisson algebra of canonical
generators form a representation of the super-BMS$_3$ algebra with an
additional central charge.
