  We report the discovery of TOI-1268b, a transiting Saturn-mass planet from
the TESS space mission. With an age of less than one Gyr, derived from various
age indicators, TOI-1268b is the youngest Saturn-mass planet known to date and
contributes to the small sample of well characterised young planets. It has an
orbital period of $P\,=\,8.1577080\pm0.0000044$ days, and transits an early K
dwarf star with a mass of $M_\star$ = $ 0.96 \pm 0.04$ $M_{\odot}$, a radius of
$R_\star$ = $ 0.92 \pm 0.06$ $R_{\odot}$, an effective temperature of
$T_\mathrm{eff}\,=\,5300\pm100$ K, and a metallicity of $0.36\pm0.06$ dex. By
combining TESS photometry with high-resolution spectra acquired with the Tull
spectrograph at McDonald observatory, and the high-resolution spectrographs at
Tautenburg and Ondrejov observatories, we measured a planetary mass of
$M_\mathrm{p}\,=\,96.4 \pm 8.3\,M_{\oplus}$ and a radius of
$R_\mathrm{p}\,=\,9.1 \pm 0.6\,R_{\oplus}$. TOI-1268 is an ideal system to
study the role of star-planet tidal interactions for non-inflated Saturn-mass
planets. We used system parameters derived in this paper to constrain the
planet tidal quality factor to the range of $10^{4.5-5.3}$. When compared with
the sample of other non-inflated Saturn-mass planets, TOI-1268b is one of the
best candidates for transmission spectroscopy studies.
