We report the spectroscopic confirmation and fundamental properties of TOI757 b, a miniNeptune on a 17.5d orbit transiting a bright star ( mag) discovered by the TESS mission. We acquired highprecision radial velocity measurements with the HARPS, ESPRESSO, and PFS spectrographs to confirm the planet detection and determine its mass. We also acquired spaceborne transit photometry with the CHEOPS space telescope to place stronger constraints on the planet radius, supported with groundbased LCOGT photometry. WASP and KELT photometry were used to help constrain the stellar rotation period. We also determined the fundamental parameters of the host star. We find that TOI757 b has a radius of and a mass of , implying a bulk density of g cm. Our internal composition modelling was unable to constrain the composition of TOI757 b, highlighting the importance of atmospheric observations for the system. We also find the planet to be highly eccentric with e = 0.39, making it one of the very few highly eccentric planets among precisely characterized miniNeptunes. Based on comparisons to other similar eccentric systems, we find a likely scenario for TOI757 b’s formation to be high eccentricity migration due to a distant outer companion. We additionally propose the possibility of a more intrinsic explanation for the high eccentricity due to starstar interactions during the earlier epoch of the Galactic disc formation, given the low metallicity and older age of TOI757.