  We present the spectroscopic confirmation and precise mass measurement of the
warm giant planet TOI-199 b. This planet was first identified in TESS
photometry and confirmed using ground-based photometry from ASTEP in Antarctica
including a full 6.5$\,$h long transit, PEST, Hazelwood, and LCO; space
photometry from NEOSSat; and radial velocities (RVs) from FEROS, HARPS,
CORALIE, and CHIRON. Orbiting a late G-type star, TOI-199\,b has a
$\mathrm{104.854_{-0.002}^{+0.001} \, d}$ period, a mass of
$\mathrm{0.17\pm0.02 \, M_J}$, and a radius of $\mathrm{0.810\pm0.005 \, R_J}$.
It is the first warm exo-Saturn with a precisely determined mass and radius.
The TESS and ASTEP transits show strong transit timing variations, pointing to
the existence of a second planet in the system. The joint analysis of the RVs
and TTVs provides a unique solution for the non-transiting companion TOI-199 c,
which has a period of $\mathrm{273.69_{-0.22}^{+0.26} \, d}$ and an estimated
mass of $\mathrm{0.28_{-0.01}^{+0.02} \, M_J}$. This period places it within
the conservative Habitable Zone.
