  Blue straggler, which are stars that appear to be younger than they should
be, are an important population of unusual stars in both stellar clusters and
the halo field of the Galaxy. Most formation scenarios evoke either stellar
collisions or binary stars that transfer mass or merge. We investigate
high-velocity stars in the Galactic halo and perform a spectral and kinematical
analysis to shed light on their nature and origin. Here we report that
SDSSJ130005.62+042201.6 (J1300+0422 for short) is an A-type star of unusually
large radial velocity (504.6 $\pm$ 5 \kms). From a quantitative NLTE (and LTE)
spectral analysis of medium-resolution optical spectra, the elemental
composition is derived. Proper motion measurements combined with a
spectroscopic distance estimate allow us to determine its present space
velocity. Its kinematical properties are derived by integrating the equation of
motion in the Galactic potential. We find J1300+0422 to be metal poor
([M/H]=$-1.2$) and exhibit an $\alpha$-element enrichment ($0.3-0.4$~dex) that
is characteristic of the halo population, as confirmed by a kinematical
analysis of its 3D space motions, which places it on a highly eccentric
retrograde Galactic orbit. The mass of J1300+0422 (1.15 $\pm$ 0.10 M$_\odot$)
is higher than the globular cluster turn-off masses indicating that it is a
halo blue straggler star. At a Galactic rest-frame velocity of
$\approx$467~\kms, the star travels faster than any known blue straggler but is
still bound to the Galaxy.
