  The Infra-Red Telescope (IRT) on board the Transient High Energy Sky and
Early Universe Surveyor (THESEUS) ESA M5 candidate mission will play a key role
in identifying and characterizing moderate to high redshift Gamma-Ray Bursts
afterglows. The IRT is the enabling instrument on board THESEUS for measuring
autonomously the redshift of the several hundreds of GRBs detected per year by
the Soft X-ray Imager (SXI) and the X- and Gamma-Ray Imaging Spectrometer
(XGIS), and thus allowing the big ground based telescopes to be triggered on a
redshift pre-selected sample, and finally fulfilling the cosmological goals of
the mission. The IRT will be composed by a primary mirror of 0.7 m of diameter
coupled to a single camera in a Cassegrain design. It will work in the 0.7-1.8
{\mu}m wavelength range, and will provide a 10x10 arc min imaging field of view
with sub-arc second localization capabilities, and, at the same time, a 5x5 arc
min field of view with moderate (R up to ~500) spectroscopic capabilities. Its
sensitivity, mainly limited by the satellite jitter, is adapted to detect all
the GRBs, localized by the SXI/XGIS, and to acquire spectra for the majority of
them.
