  We present analysis of a new pulsating helium-atmosphere (DB) white dwarf,
EPIC~228782059, discovered from 55.1~days of {\em K2} photometry. The long
duration, high quality light curves reveal 11 independent dipole and quadruple
modes, from which we derive a rotational period of $34.1 \pm 0.4$~hr for the
star. An optimal model is obtained from a series of grids constructed using the
White Dwarf Evolution Code, which returns $M_{*} = 0.685 \pm 0.003 M_{\odot}$,
$T_{\rm{eff}}= 21{,}910 \pm 23$\,K and $\log g = 8.14 \pm0.01$\,dex. These
values are comparable to those derived from spectroscopy by Koester \& Kepler
($20{,}860 \pm 160$\,K and $7.94 \pm0.03$\,dex). If these values are confirmed
or better constrained by other independent works, it would make EPIC~228782059
one of the coolest pulsating DB white dwarf star known, and would be helpful to
test different physical treatments of convection, and to further investigate
the theoretical instability strip of DB white dwarf stars.
