  The supersymmetry invariance of flat supergravity (i.e., supergravity in the
absence of any internal scale in the Lagrangian) in four dimensions on a
manifold with non-trivial boundary is explored. Using a geometric approach we
find that the supersymmetry invariance of the Lagrangian requires to add
appropriate boundary terms. This is achieved by considering additional gauge
fields to the boundary without modifying the bulk Lagrangian. We also construct
an enlarged supergravity model from which, in the vanishing cosmological
constant limit, flat supergravity with a non-trivial boundary emerges properly.
