<title>Abstract</title>
We report the discovery of an ultrahot Jupiter with an extremely short orbital period of 0.67247414 ± 0.00000028 days (∼16 hr). The 1.347 ± 0.047 <italic>R</italic>
_Jup planet, initially identified by the Transiting Exoplanet Survey Satellite (TESS) mission, orbits TOI-2109 (TIC 392476080)—a <italic>T</italic>
_eff ∼ 6500 K F-type star with a mass of 1.447 ± 0.077 <italic>M</italic>
_☉, a radius of 1.698 ± 0.060 <italic>R</italic>
_☉, and a rotational velocity of <inline-formula>
<tex-math>
<?CDATA $v\sin {i}_{* }=81.9\pm 1.7$?>
</tex-math>
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
<mml:mi>v</mml:mi>
<mml:mi>sin</mml:mi>
<mml:msub>
<mml:mrow>
<mml:mi>i</mml:mi>
</mml:mrow>
<mml:mrow>
<mml:mo>*</mml:mo>
</mml:mrow>
</mml:msub>
<mml:mo>=</mml:mo>
<mml:mn>81.9</mml:mn>
<mml:mo>±</mml:mo>
<mml:mn>1.7</mml:mn>
</mml:math>
<inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ajac26bdieqn1.gif" xlink:type="simple" />
</inline-formula> km s⁻¹. The planetary nature of TOI-2109b was confirmed through radial-velocity measurements, which yielded a planet mass of 5.02 ± 0.75 <italic>M</italic>
_Jup. Analysis of the Doppler shadow in spectroscopic transit observations indicates a well-aligned system, with a sky-projected obliquity of <italic>λ</italic> = 1.°7 ± 1.°7. From the TESS full-orbit light curve, we measured a secondary eclipse depth of 731 ± 46 ppm, as well as phase-curve variations from the planet’s longitudinal brightness modulation and ellipsoidal distortion of the host star. Combining the TESS-band occultation measurement with a <italic>K</italic>

_{<italic>s</italic>}
-band secondary eclipse depth (2012 ± 80 ppm) derived from ground-based observations, we find that the dayside emission of TOI-2109b is consistent with a brightness temperature of 3631 ± 69 K, making it the second hottest exoplanet hitherto discovered. By virtue of its extreme irradiation and strong planet–star gravitational interaction, TOI-2109b is an exceptionally promising target for intensive follow-up studies using current and near-future telescope facilities to probe for orbital decay, detect tidally driven atmospheric escape, and assess the impacts of H₂ dissociation and recombination on the global heat transport.