We report the detection of transits by the 3.1 M_Jup companion to the V=8.17 G0V star HD 17156. The transit was observed by three independant observers on Sep. 9/10, 2007 (two in central Italy and one in the Canary Islands), who obtained detections at confidence levels of 3.0σ, 5.3σ, and 7.9σ, respectively. The observations were carried out under the auspices of the Transitsearch.org network, which organizes follow-up photometric transit searches of known planet-bearing stars during the time intervals when transits might be expected to occur. Analyses of the 7.9σ data set indicates a transit depth d=0.0062 ± 0.0004 and a transit duration t=186 ± 5 min. These values are consistent with the transit of a Jupiter-sized planet with an impact parameter b=a cos{i}/Rstar ˜ 0.8. This planet occupies a unique regime among known transiting extrasolar planets, both as a result of its large orbital eccentricity (e=0.67) and long orbital period (P=21.2 d). The planet receives a 26-fold variation in insolation during the course of its orbit, which will make it a useful object for characterizing exoplanetary atmospheric dynamics.