Authors:Barragán et alAbstract:We report the discovery of EPIC 211391664b, 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. EPIC 211391664 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±0.042M⊙, R⋆=1.311+0.083−0.048R⊙, and age of 5.2+1.2−1.0~Gyr. We derive a planetary mass and radius of Mp=32.2±8.1M⊕ and Rp=4.3+0.3−0.2R⊕. EPIC 211391664b joins the relatively small group of Neptune-sized planets whose mass and radius have been derived with a precision better than 3-σ. We estimate that the planet will be engulfed by EPIC 211391664 in ∼3~Gyr, due to the evolution of the host star towards the red giant branch.