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 $M_star$=$ 1.074pm0.042$, $R_star$=$ 1.311 ^{+ 0.083} _{ - 0.048} $, and age of $5.2_{-1.0}^{+1.2}$~Gyr. We derive a planetary mass and radius of $M_mathrm{p}$=$ 32.2 pm 8.1 $ and $R_mathrm{p}$=$4.3^{+0.3}_{-0.2}$. 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 $sim$3~Gyr, due to the evolution of the latter towards the red giant branch.
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