White dwarfs accreting planetary debris provide detailed insight into the bulk composition of rocky exoplanetesimals. However, only one Kuiper Belt analogue has been identified in that way so far. Here, we report the accretion of an icy extrasolar planetesimal on to white dwarf WD 1647+375 using ultraviolet spectroscopy from the Hubble Space Telescope. The accreted material is rich in the volatiles carbon, nitrogen, and sulphur, with a chemical composition analogous to Kuiper belt objects (KBOs) in our Solar system. It has a high nitrogen mass fraction ($5.1\pm 1.6$ per cent) and large oxygen excess ($84\pm 7$ per cent), indicating that the accreted planetesimal is water-rich (a water-to-rock ratio of ${\simeq} 2.45$), corroborating a cometary- or dwarf planet-like composition. The white dwarf has been accreting at a rate of ${\approx} 2\times 10^{8}\, \mbox{$\mathrm{g\, s^{-1}}$}$ for the past 13 yrs, implying a minimum mass of ${\sim} 10^{17}$ g for the icy parent body. The actual mass could be several orders of magnitude larger if the accretion phase lasts ${\sim} 10^5$ yr as estimated in the literature from debris disc studies. We argue that the accreted body is most likely a fragment of a KBO dwarf planet based on its nitrogen-rich composition. However, based on the chemical composition alone, it is difficult to discern whether this icy body is intrinsic to this planetary system, or may have an interstellar origin.