Authors:Barragán et alAbstract:We report the discovery of K2-98 b (EPIC 211391664 b), a transiting Neptune-sized planet monitored by the K2 mission during its campaign 5. We combine the K2 time-series data with ground-based photometric and spectroscopic follow-up observations to confirm the planetary nature of the object and derive its mass, radius, and orbital parameters. K2-98 b is a warm Neptune-like planet in a 10-day orbit around a V=12.2~mag F-type star with M⋆=1.074±0.042, R⋆=1.311+0.083−0.048, and age of 5.2+1.2−1.0~Gyr. We derive a planetary mass and radius of Mp=32.2±8.1 and Rp=4.3+0.3−0.2. K2-98 b joins the relatively small group of Neptune-sized planets whose both mass and radius have been derived with a precision better than 25 %. We estimate that the planet will be engulfed by its host star in ∼3~Gyr, due to the evolution of the latter towards the red giant branch.