  We report the discovery of a spectacular ``post-starburst quasar'' UN
J1025-0040 (B=19; z=0.634). The optical spectrum is a chimera, displaying the
broad Mg II emission line and strong blue continuum characteristic of quasars,
but is dominated in the red by a large Balmer jump and prominent high-order
Balmer absorption lines indicative of a substantial young stellar population at
similar redshift. Stellar synthesis population models show that the stellar
component is consistent with a 400 Myr old instantaneous starburst with a mass
of less than or about 10^{11} solar masses. A deep, K_s-band image taken in 0.5
arcsec seeing shows a point source surrounded by asymmetric extended fuzz.
Approximately 70% of the light is unresolved, the majority of which is expected
to be emitted by the starburst. While starbursts and galaxy interactions have
been previously associated with quasars, no quasar ever before has been seen
with such an extremely luminous young stellar population.
