  We use TESS, Spitzer, ground-based light curves and HARPS spectrograph radial
velocity measurements to establish the physical properties of the transiting
exoplanet candidate TOI-674b. We perform a joint fit of the light curves and
radial velocity time series to measure the mass, radius, and orbital parameters
of the candidate. We confirm and characterize TOI-674b, a low-density
super-Neptune transiting a nearby M dwarf. The host star (TIC 158588995, $V =
14.2$ mag, $J = 10.3$ mag) is characterized by its M2V spectral type with
$\mathrm{M}_\star=0.420\pm 0.010$ M$_\odot$, $\mathrm{R}_\star = 0.420\pm
0.013$ R$_\odot$, and $\mathrm{T}_{\mathrm{eff}} = 3514\pm 57$ K, and is
located at a distance $d=46.16 \pm 0.03$ pc. Combining the available transit
light curves plus radial velocity measurements and jointly fitting a circular
orbit model, we find an orbital period of $1.977143 \pm 3\times 10^{-6}$ days,
a planetary radius of $5.25 \pm 0.17$ $\mathrm{R}_\oplus$, and a mass of $23.6
\pm 3.3$ $\mathrm{M}_\oplus$ implying a mean density of $\rho_\mathrm{p} = 0.91
\pm 0.15$ [g cm$^{-3}$]. A non-circular orbit model fit delivers similar
planetary mass and radius values within the uncertainties. Given the measured
planetary radius and mass, TOI-674b is one of the largest and most massive
super-Neptune class planets discovered around an M type star to date. It is
also a resident of the so-called Neptunian desert and a promising candidate for
atmospheric characterisation using the James Webb Space Telescope.
