  We present the discovery of a highly irradiated and moderately inflated
ultra-hot Jupiter, TOI-1431b/MASCARA-5b (HD 201033b), first detected by NASA's
Transiting Exoplanet Survey Satellite mission (TESS) and the Multi-site 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 $K=294.1\pm1.1$ 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 $M_{p}=3.12\pm0.18$
$\rm{M_J}$ ($990\pm60$ M$_{\oplus}$), an inflated radius of $R_{p}=1.49\pm0.05$
$\rm{R_J}$ ($16.7\pm0.6$ R$_{\oplus}$), and an orbital period of
$P=2.650237\pm0.000003$ d. Analysis of the spectral energy distribution of the
host star reveals that the planet orbits a bright ($\mathrm{V}=8.049$ mag) and
young ($0.29^{+0.32}_{-0.19}$ Gyr) Am type star with $T_{\rm
eff}=7690^{+400}_{-250}$ $\rm{K}$, resulting in a highly irradiated planet with
an incident flux of $\langle F \rangle=7.24^{+0.68}_{-0.64}\times$10$^9$ erg
s$^{-1}$ cm$^{-2}$ ($5300^{+500}_{-470}\mathrm{S_{\oplus}}$) and an equilibrium
temperature of $T_{eq}=2370\pm70$ K. TESS photometry also reveals a secondary
eclipse with a depth of $127^{+4}_{-5}$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
$T_\mathrm{day}=3004\pm64$ K and $T_\mathrm{night}=2583\pm63$ K, the second
hottest measured nightside temperature. The planet's low day/night temperature
contrast ($\sim$420 K) suggests very efficient heat transport between the
dayside and nightside hemispheres.
