  We report the discovery of TOI 837b and its validation as a transiting
planet. We characterize the system using data from the NASA TESS mission, the
ESA Gaia mission, ground-based photometry from El Sauce and ASTEP400, and
spectroscopy from CHIRON, FEROS, and Veloce. We find that TOI 837 is a $T=9.9$
mag G0/F9 dwarf in the southern open cluster IC 2602. The star and planet are
therefore $35^{+11}_{-5}$ million years old. Combining the transit photometry
with a prior on the stellar parameters derived from the cluster color-magnitude
diagram, we find that the planet has an orbital period of $8.3\,{\rm d}$ and is
slightly smaller than Jupiter ($R_{\rm p} = 0.77^{+0.09}_{-0.07} \,R_{\rm
Jup}$). From radial velocity monitoring, we limit $M_{\rm p}\sin i$ to less
than 1.20 $M_{\rm Jup}$ (3-$\sigma$). The transits either graze or nearly graze
the stellar limb. Grazing transits are a cause for concern, as they are often
indicative of astrophysical false positive scenarios. Our follow-up data show
that such scenarios are unlikely. Our combined multi-color photometry,
high-resolution imaging, and radial velocities rule out hierarchical eclipsing
binary scenarios. Background eclipsing binary scenarios, though limited by
speckle imaging, remain a 0.2% possibility. TOI 837b is therefore a validated
adolescent exoplanet. The planetary nature of the system can be confirmed or
refuted through observations of the stellar obliquity and the planetary mass.
Such observations may also improve our understanding of how the physical and
orbital properties of exoplanets change in time.
