  In spite of unscreened Coulomb interactions close to charge neutrality,
relativistic massless electrons in graphene allegedly behave as noninteracting
particles. A clue to this paradox is that both interaction and kinetic energies
scale with particle density in the same way. In contrast, in a dilute gas of
nonrelativistic electrons the different scaling drives the transition to Wigner
crystal. Here we show that Dirac electrons in a graphene quantum dot with a
mass gap localize \`a la Wigner for realistic values of device parameters. Our
theoretical evidence relies on many-body observables obtained through the exact
diagonalization of the interacting Hamiltonian, which allows us to take all
electron correlations into account. We predict that the experimental signatures
of Wigner localization are the suppression of the fourfold periodicity of the
filling sequence and the quenching of excitation energies, which may be both
accessed through Coulomb blockade spectroscopy. Our findings are relevant to
other carbon-based nanostructures exhibiting a mass gap.
