We present the discovery and validation of TOI-7166 b, a $2.01 \pm 0.05\,\mathrm{ R}_{\oplus }$ planet orbiting a nearby low-mass star. We validated the planet by combining Transiting Exoplanet Survey Satellite and multicolour high-precision photometric observations from ground-based telescopes, together with spectroscopic data, high-contrast imaging, archival images, and statistical arguments. The host star is an M4-type dwarf at a distance of $\sim$35 pc from the Sun. It has a mass and a radius of $M_\star = 0.190 \pm 0.004\, \mathrm{ M}_\odot$ and $R_\star =0.222 \pm 0.005\, \mathrm{ R}_\odot$, respectively. TOI-7166 b has an orbital period of 12.9 d, which places it close to the inner edge of the Habitable Zone of its host star, receiving an insolation flux of $S_\mathrm{ p}=1.07\pm 0.08\, \mathrm{ S}_{\oplus }$ and an equilibrium temperature of $T_{\rm eq}~= 249\pm 5$ K (assuming a null Bond albedo). The brightness of the host star makes TOI-7166 a suitable target for radial velocity follow-up to measure the planetary mass and bulk density. Moreover, the physical parameters of the system including the infrared brightness ($K_{\rm mag} = 10.6$) of the star and the planet-to-star radius ratio ($0.0823 \pm 0.0012$) make TOI-7166 b an exquisite target for transmission spectroscopic observations with the James Webb Space Telescope, to constrain the exoplanet atmospheric compositions.