  The recent discovery of the electromagnetic counterpart of the gravitational
wave source GW170817 has demonstrated the huge informative power of
multi-messenger observations. Late '20s and early '30s will be a mature era for
multi-messenger astronomy. Consolidated network of second generation
gravitational wave detectors, such as Advanced LIGO and Advanced Virgo, KAGRA
and LIGO-India, will be further powered by the contribution from third
generation interferometers such as Einstein Telescope and/or Cosmic Explorer.
Several astrophysical sources detectable in GWs are expected to radiate in the
full electromagentic spectrum and to emit high energy neutrinos, thus requiring
a robust synergy with ground- and space-based high energy detectors (e.g. CTA,
THESEUS, ATHENA), sensitive neutrino detectors (e.g. KM3Net, IceCube-Gen2) and
large size optical facilities (e.g. E-ELT). In this report we review the
fundamental role of THESEUS in this exciting context.
