We present a search for the proton decay mode p ->(v) over barK(+) based on an exposure of 8.97 kton-years in the KamLAND experiment. The liquid scintillator detector is sensitive to successive signals from p ->(v) over barK(+) with unique kinematics, which allow us to achieve a detection efficiency of 44%, higher than previous searches in water Cherenkov detectors. We find no evidence of proton decays for this mode. The expected background, which is dominated by atmospheric neutrinos, is 0.9 +/- 0.2 events. The nonbackground-subtracted limit on the partial proton lifetime is tau/B (p ->(v) over barK(+)) > 5.4 x 10(32) years at 90% C. L.