  The recent discoveries of Neptune-sized ultra-short period planets (USPs)
challenge existing planet formation theories. It is unclear whether these
residents of the Hot Neptune Desert have similar origins to smaller, rocky
USPs, or if this discrete population is evidence of a different formation
pathway altogether. We report the discovery of TOI-3261b, an ultra-hot Neptune
with an orbital period $P$ = 0.88 days. The host star is a $V = 13.2$
magnitude, slightly super-solar metallicity ([Fe/H] $\simeq$ 0.15), inactive
K1.5 main sequence star at $d = 300$ pc. Using data from the Transiting
Exoplanet Survey Satellite and the Las Cumbres Observatory Global Telescope, we
find that TOI-3261b has a radius of $3.82_{-0.35}^{+0.42}$ $R_{\oplus}$.
Moreover, radial velocities from ESPRESSO and HARPS reveal a mass of
$30.3_{-2.4}^{+2.2}$ $M_{\oplus}$, more than twice the median mass of
Neptune-sized planets on longer orbits. We investigate multiple mechanisms of
mass loss that can reproduce the current-day properties of TOI-3261b,
simulating the evolution of the planet via tidal stripping and
photoevaporation. Thermal evolution models suggest that TOI-3261b should retain
an envelope potentially enriched with volatiles constituting $\sim$5% of its
total mass. This is the second highest envelope mass fraction among ultra-hot
Neptunes discovered to date, making TOI-3261b an ideal candidate for
atmospheric follow-up observations.
