  THESEUS is an ESA space based project, aiming to explore the early universe
by unveiling a complete census of Gamma-Ray Burst (GRB) population in the first
billion years. This goal is expected to be achieved by combined observations of
its three instruments: the Soft X-ray Imager (SXI), the X and Gamma Imaging
Spectrometer (XGIS) and the InfraRed Telescope (IRT). In particular, the IRT
instrument will help to identify, localise and study the afterglow of the GRBs
detected by SXI and XGIS, and about $40\%$ of its time will be devoted to an
all-sky photometric survey, which will certainly detect a relevant number of
extragalactic sources, including Quasars. In this paper, we focus on the
capability of IRT-THESEUS Telescope to observe Quasars and, in particular,
those objects lensed by foreground galaxies. In our analysis, we consider the
Quasar Luminosity Function (QLF) in the infrared band based obtained by the
Spitzer Space Telescope imaging survey. Furthermore, by using the
mass-luminosity distribution function of galaxies and the galaxy/Quasar
redshift distributions, we preformed Monte Carlo simulations to estimate the
number of lensed Quasars. We predict that up to $2.14 \times 10^5$ Quasars can
be observed during gthe IRT-Theseus sky survey, and about $140$ of them could
be lensed by foreground galaxies. Detailed studies of these events would
provide a powerful probe of the physical properties of Quasars and the mass
distribution models of the galaxies.
