  We report the discovery of a quasar at z=3.050+/-0.003, closely coincident in
redshift with the isolated low-opacity feature seen near z~3.056 in the
otherwise black portion of the HeII Gunn-Peterson absorption trough seen toward
the z=3.286 background quasar Q0302-003, located 6.5' away on the sky. We
explore plausible models for the HeIII ionization zone created by this
neighboring quasar and its interception with the line of sight toward
Q0302-003. At its present brightness of V=20.5 and separation of D=3.2 Mpc, the
quasar can readily account for the opacity gap in the HeII absorption spectrum
of Q0302-003, provided it has been active for t_Q > 10^7 y. This is the first
clear detection of the `transverse' proximity effect and imprint of a quasar on
the intervening absorption detected along an adjacent line of sight.
