  We validate the Transiting Exoplanet Survey Satellite (TESS) object of
interest TOI-2266.01 (TIC 348911) as a small transiting planet (most likely a
super-Earth) orbiting a faint M5 dwarf ($V=16.54$) on a 2.33~d orbit. The
validation is based on an approach where multicolour transit light curves are
used to robustly estimate the upper limit of the transiting object's radius.
Our analysis uses SPOC-pipeline TESS light curves from Sectors 24, 25, 51, and
52, simultaneous multicolour transit photometry observed with MuSCAT2, MuSCAT3,
and HiPERCAM, and additional transit photometry observed with the LCOGT
telescopes. TOI-2266 b is found to be a planet with a radius of
$1.54\pm\0.09\,R_\oplus$, which locates it at the edge of the transition zone
between rocky planets, water-rich planets, and sub-Neptunes (the so-called
M~dwarf radius valley). The planet is amenable to ground-based radial velocity
mass measurement with red-sensitive spectrographs installed in large
telescopes, such as MAROON-X and Keck Planet Finder (KPF), which makes it a
valuable addition to a relatively small population of planets that can be used
to probe the physics of the transition zone. Further, the planet's orbital
period of 2.33 days places it inside a `keystone planet' wedge in the
period-radius plane where competing planet formation scenarios make conflicting
predictions on how the radius valley depends on the orbital period. This makes
the planet also a welcome addition to the small population of planets that can
be used to test small-planet formation scenarios around M~dwarfs.
