We present the discovery of a highly irradiated and moderately inflated ultrahot Jupiter, TOI-1431b/MASCARA5 b (HD 201033b), first detected by NASA’s Transiting Exoplanet Survey Satellite mission (TESS) and the Multisite All-Sky Camera (MASCARA). The signal was established to be of planetary origin through radial velocity measurements obtained using SONG, SOPHIE, FIES, NRES, and EXPRES, which show a reflex motion of K = 294.1 ± 1.1 m s−1. A joint analysis of the TESS and ground-based photometry and radial velocity measurements reveals that TOI-1431b has a mass of Mp = 3.12 ± 0.18 MJ (990 ± 60 M⊕), an inflated radius of Rp = 1.49 ± 0.05 RJ (16.7 ± 0.6 R⊕), and an orbital period of P = 2.650237 ± 0.000003 days. Analysis of the spectral energy distribution of the host star reveals that the planet orbits a bright (V = 8.049 mag) and young ( -+ 0.29 0.19 0.32 Gyr) Am type star with = -+ Teff 7690 250 400 K, resulting in a highly irradiated planet with an incident flux of á ñ= ´ - + F 7.24 0.64 0.68 109 erg s−1 cm−2 ( - + 5300 470 SÅ 500 ) and an equilibrium temperature of Teq = 2370 ± 70 K. TESS photometry also reveals a secondary eclipse with a depth of - + 127 5 4 ppm as well as the full phase curve of the planet’s thermal emission in the red-optical. This has allowed us to measure the dayside and nightside temperature of its atmosphere as Tday = 3004 ± 64 K and Tnight = 2583 ± 63 K, the second hottest measured nightside temperature. The planet’s low day/night temperature contrast (∼420 K) suggests very efficient heat transport between the dayside and nightside hemispheres. Given the host star brightness and estimated secondary eclipse depth of ∼1000 ppm in the K band, the secondary eclipse is potentially detectable at near-IR wavelengths with ground-based facilities, and the planet is ideal for intensive atmospheric characterization through transmission and emission spectroscopy from space missions such as the James Webb Space Telescope and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey.