We present the confirmation of a new sub-Neptune close to the transition between Super-Earths and sub-Neptunes transiting the M2 dwarf TOI-269. The exoplanet candidate is identified in multiple TESS sectors and is validated with high-precision spectroscopy from HARPS and ground-based photometric follow-up from ExTrA and LCO-CTIO. We determine mass, radius and bulk density of the exoplanet by jointly modeling both photometry and radial velocities with juliet. The transiting exoplanet has an orbital period of P=3.7 days, a radius of 2.77+/-0.12R_{Earth}_, and a mass of 8.8+/-1.4M_{Earth}_. Since TOI-269 b lies among the best targets of its category for atmospheric characterization, it would be interesting to probe the atmosphere of this exoplanet with transmission spectroscopy in order to compare it to other sub-Neptunes. With an eccentricity e=0.425^+0.082^_-0.086_, TOI-269 b has one of the highest eccentricity among exoplanets with periods less than 10 days. The star being likely a few Gyr old, this system does not appear to be dynamically young. We surmise TOI-269 b may have acquired a high eccentricity as it migrated inward through planet-planet interactions.