On the basis of astrometric and spectroscopic data we argue that the ultramassive white dwarf GD 50 is associated with the star formation event that created the Pleiades and is potentially a former cluster member. Its cooling time (˜60Myrs) is consistent with it having evolved essentially as a single star from a M>6M&sun; progenitor so there appears to be no need to invoke a white dwarf--white dwarf binary merger scenario to account for its existence. Our result may represent the first direct observational evidence that single star evolution can produce white dwarfs with M>1.1M&sun;, as predicted by some stellar evolutionary theories. Additionally, our findings may help towards alleviating the difficulties in reconciling the observed number of hot nearby ultramassive white dwarfs with the smaller number predicted by binary evolution models under the assumption that they are the products of white dwarf mergers.