  We report the discovery and validation of HD 21520 b, a transiting planet
found with TESS and orbiting a bright G dwarf (V=9.2, $T_{eff} = 5871 \pm 62$
K, $R_{\star} = 1.04\pm 0.02\, R_{\odot}$). HD 21520 b was originally alerted
as a system (TOI-4320) consisting of two planet candidates with periods of
703.6 and 46.4 days. However, our analysis supports instead a single-planet
system with an orbital period of $25.1292\pm0.0001$ days and radius of $2.70
\pm 0.09\, R_{\oplus}$. Three full transits in sectors 4, 30 and 31 match this
period and have transit depths and durations in agreement with each other, as
does a partial transit in sector 3. We also observe transits using CHEOPS and
LCOGT. SOAR and Gemini high-resolution imaging do not indicate the presence of
any nearby companions, and MINERVA-Australis and CORALIE radial velocities rule
out an on-target spectroscopic binary. Additionally, we use ESPRESSO radial
velocities to obtain a tentative mass measurement of $7.9^{+3.2}_{-3.0}\,
M_{\oplus}$, with a 3-$\sigma$ upper limit of 17.7 $M_{\oplus}$. Due to the
bright nature of its host and likely significant gas envelope of the planet, HD
21520 b is a promising candidate for further mass measurements and for
atmospheric characterization.
