We present a measurement of the direct CP violating asymmetry in the decay B0→K+π- using a data sample of 227×10^6 Υ(4S)→BB̅ decays collected with the BABAR detector at the PEP-II asymmetric-energy e+e- collider at SLAC. We observe a total signal yield of nK-π++nK+π-=1606±51 decays and measure the asymmetry (nK-π+-nK+π-)/(nK-π++nK+π-)=-0.133±0.030(stat)±0.009(syst). The probability of observing such an asymmetry in the absence of direct CP violation is 1.3×10^-5, corresponding to 4.2 standard deviations.
