The design and the performance of the K3 beam at the KEK 12 GeV Proton Synchrotron are reported. The beam line provides separated kaons up to a maximum momentum of 0.8 GeV/c and separated antiprotons up to 1.1 GeV/c. The beam can be operated in two different modes: a high-intensity mode, K3-S, and a high-momentum-resolution mode, K3-L. The solid-angle momentum acceptance and the beam length of K3-S are 48.4 msr Δ PP and 14.45 m, respectively, and those of K3-L are 13.3 msr%ΔPP and 16.3 m. Measured optical parameters of K3-S agreed with the designed values. Measured intensity of K+ per 1012 protons and the π+μ+e+K ratio were 1.96 × 104 and 4.2 at 500 MeV/c, 3.12 × 104 and 3.0 at 540 MeV/c, and 5.18 × 104 and 2.6 at 600 MeV/c.