  We report the discovery of a 1.32$^{+0.10}_{-0.10}$ $\mathrm{M_{\rm Jup}}$
planet orbiting on a 75.12 day period around the G3V $10.8^{+2.1}_{-3.6}$ Gyr
old star TOI-5542 (TIC 466206508; TYC 9086-1210-1). The planet was first
detected by the Transiting Exoplanet Survey Satellite (TESS) as a single
transit event in TESS Sector 13. A second transit was observed 376 days later
in TESS Sector 27. The planetary nature of the object has been confirmed by
ground-based spectroscopic and radial velocity observations from the CORALIE
and HARPS spectrographs. A third transit event was detected by the ground-based
facilities NGTS, EulerCam, and SAAO. We find the planet has a radius of
1.009$^{+0.036}_{-0.035}$ $\mathrm{R_{\rm Jup}}$ and an insolation of
9.6$^{+0.9}_{-0.8}$ $S_{\oplus}$, along with a circular orbit that most likely
formed via disk migration or in situ formation, rather than high-eccentricity
migration mechanisms. Our analysis of the HARPS spectra yields a host star
metallicity of [Fe/H] = $-$0.21$\pm$0.08, which does not follow the traditional
trend of high host star metallicity for giant planets and does not bolster
studies suggesting a difference among low- and high-mass giant planet host star
metallicities. Additionally, when analyzing a sample of 216 well-characterized
giant planets, we find that both high masses (4 $\mathrm{M_{\rm Jup}}$
$<M_{p}<$ 13 $\mathrm{M_{\rm Jup}}$) and low masses (0.5 $\mathrm{M_{\rm Jup}}$
$<M_{p}<$ 4 $\mathrm{M_{\rm Jup}}$), as well as both both warm (P $>$ 10 days)
and hot (P $<$ 10 days) giant planets are preferentially located around
metal-rich stars (mean [Fe/H] $>$ 0.1). TOI-5542b is one of the oldest known
warm Jupiters and it is cool enough to be unaffected by inflation due to
stellar incident flux, making it a valuable contribution in the context of
planetary composition and formation studies.

Earth and Planetary Astrophysics