The tt spin correlation at production is a fundamental prediction of QCD and a potentially incisive test of new physics coupled to top quarks. We measure the tt spin state in pp collisions at s√=1.96  TeV using 1001 candidate events in the lepton plus jets decay channel reconstructed in the CDF II detector. In the helicity basis, for a top-quark mass of 172.5  GeV/c2, we find a spin correlation coefficient κ=0.60±0.50  (stat)±0.16  (syst), consistent with the QCD prediction, κ≈0.40.