  Multi-messenger astrophysics is becoming a major avenue to explore the
Universe, with the potential to span a vast range of redshifts. The growing
synergies between different probes is opening new frontiers, which promise
profound insights into several aspects of fundamental physics and cosmology. In
this context, THESEUS will play a central role during the 2030s in detecting
and localizing the electromagnetic counterparts of gravitational wave and
neutrino sources that the unprecedented sensitivity of next generation
detectors will discover at much higher rates than the present. Here, we review
the most important target signals from multi-messenger sources that THESEUS
will be able to detect and characterize, discussing detection rate expectations
and scientific impact.
