Résumé
The Cherenkov Telescope Array (CTA) is designed to be the next major observatory operating in the Very High Energy (VHE, $\gtrsim 100$ GeV) gamma-ray band. It will build on the imaging atmospheric Cherenkov technique but will go much further in terms of performance than current instruments. Its sensitivity at short timescales and the rapid repointing system will make CTA a perfect facility to observe the gamma-ray transient sky. In this respect, the synergies between CTA and other multi-wavelength and multi-messenger facilities are expected to further enhance CTA's scientific scope. Thanks to its characteristics, THESEUS will perform an unprecedented monitoring of the X-ray variable sky, detecting, localising, and identifying transients. It will provide external triggers and accurate location for the CTA follow-up of transients, and their broadband characterisation, playing a key role for CTA after the next decade.