In the ongoing HATNet survey we have detected a giant planet, with radius 1.33 plus or minus 0.06 R sub(Jup) and mass 1.06 plus or minus 0.12 M sub(Jup), transiting the bright (V = 10.5) star GSC 03239-00992. The planet is in a circular orbit with period 3.852985 plus or minus 0.000005 days and midtransit epoch 2,454,035.67575 plus or minus 0.00028 (HJD). The parent star is a late F star with mass 1.29 plus or minus 0.06 M unk, radius 1.46 plus or minus 0.06 R unk, T sub(off) similar to 6570 plus or minus 80 K, Fe/H = -0.13 plus or minus 0.08, and age similar to 2.3 super(+) sub(-) super(0) sub(0) super(.) sub(.) super(5) sub(7) Gyr. With this radius and mass, HAT-P-6b has somewhat larger radius than theoretically expected. We describe the observations and their analysis to determine physical properties of the HAT-P-6 system, and briefly discuss some implications of this finding.