  We study the prospects of future gravitational wave (GW) detectors in probing
primordial black hole (PBH) binaries. We show that across a broad mass range
from $10^{-5}M_\odot$ to $10^7M_\odot$, future GW interferometers provide a
potential probe of the PBH abundance that is more sensitive than any currently
existing experiment. In particular, we find that galactic PBH binaries with
masses as low as $10^{-5}M_\odot$ may be probed with ET, AEDGE and LISA by
searching for nearly monochromatic continuous GW signals. Such searches could
independently test the PBH interpretation of the ultrashort microlensing events
observed by OGLE. We also consider the possibility of observing GWs from
asteroid mass PBH binaries through graviton-photon conversion.
