  The Transiting Exoplanet Survey Satellite mission identified a deep and
asymmetric transit-like signal with a periodicity of 4.5 days orbiting the M4
dwarf star TOI-3884. The signal has been confirmed by follow-up observations
collected by the ExTrA facility and Las Cumbres Observatory Global Telescope,
which reveal that the transit is chromatic. The light curves are well modelled
by a host star having a large polar spot transited by a 6-R$_{\oplus}$ planet.
We validate the planet with seeing-limited photometry, high-resolution imaging,
and radial velocities. TOI-3884 b, with a radius of $6.00 \pm 0.18$
R$_{\oplus}$, is the first sub-Saturn planet transiting a mid-M dwarf. Owing to
the host star's brightness and small size, it has one of the largest
transmission spectroscopy metrics for this planet size and becomes a top target
for atmospheric characterisation with the James Webb Space Telescope and
ground-based telescopes.
