  We explore the mono-$Z$ signature for dark matter searches at future high
energy $e^+e^-$ colliders. In the context of effective field theory, we
consider two kinds of contact operators describing dark matter interactions
with electroweak gauge bosons and with electron/positron, respectively. For
five benchmark models, we propose kinematic cuts to distinguish signals from
backgrounds for both charged leptonic and hadronic decay modes of the $Z$
boson. We also present the experimental sensitivity to cutoff scales of
effective operators and compare it with that of the Fermi-LAT indirect search
and demonstrate the gains in significance for the several configurations of
polarized beams.
