We report the detection of a transiting Jupiter-sized planet orbiting a relatively bright (V=11.79) K0 V star. We detected the transit light-curve signature in the course of the TrES multisite transiting planet survey and confirmed the planetary nature of the companion via multicolor photometry and precise radial velocity measurements. We designate the planet TrES-1 its inferred mass is (0.75+/-0.07)MJup, its radius is 1.08+0.18-0.04RJup, and its orbital period is 3.030065+/-0.000008 days. This planet has an orbital period similar to that of HD 209458b but about twice as long as those of the OGLE transiting planets. Its mass is indistinguishable from that of HD 209458b, but its radius is significantly smaller and fits the theoretical models without the need for an additional source of heat deep in the atmosphere, as has been invoked by some investigators for HD 209458b.