  In this letter we point out that the Belle II experiment has a unique
sensitivity to visibly decaying long-lived dark photons. Concentrating on the
signatures with a single high energy photon in association with a displaced
pair of charged particles, we find that Belle II will be able to probe large
regions of parameter space that cannot be covered by any other running or
proposed experimental facility. While the signature with charged muons or pions
in the final state is expected to be background-free after all selections are
applied, the case of final state electrons is more involved and requires an
in-depth study. We discuss possible ways to further suppress backgrounds and
the corresponding experimental prospects.
