We report an experimental demonstration of single-photon switching in laser-cooled ^87Rb atoms. A resonant probe pulse with an energy per unit area of one photon per λ^2/2π propagates through the optically thick atoms. Its energy transmittance is greater than 63 e^-1 due to the effect of electromagnetically induced transparency. In the presence of a switching pulse with an energy per unit area of 1.4 photons per λ^2/2π, the energy transmittance of the same probe pulse becomes less than 37

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... by single switching photons has potential applications in single-photon-level nonlinear optics and the manipulation of quantum information.