We report the discovery of a new hydrogen atmosphere variable white dwarf (DAV; ZZ Ceti), G30-20, a high-amplitude, long-period pulsator. The Fourier transform of its light curve shows power around 1050-1125 s. We suspect the existence of multiple periodicities in this region, probably corresponding to different radial order (k) modes, but our observations are not long enough to resolve them. This discovery increases the number of known DAVs to 32. Our best fit to its optical spectrum gives Teff = 11,070 ± 180 K and log g = 7.95 ± 0.06. This implies that G30-20 is the coolest known DAV.