Until now, evidence for lightning on Saturn has been indirect – through radio emissions and cloud morphology. Here we report the first visible detection of lightning, on the night side on August 17, 2009 at −36.4° ± 0.1° planetocentric latitude and 10.6° ± 0.9° west longitude. No other locations produced lightning detectable by either imaging or radio. The lightning images are consistent with a single cloud flashing once per minute. The visible energy of a single flash is comparable to that on Earth and Jupiter, and ranges up to 1.7 × 109 Joules. The diameter of the lightning flashes is ∼200 km, which suggests the lightning is 125–250 km below cloud tops. This depth is above the base of the liquid H2O-NH3 cloud and may be either in the NH4SH cloud or in the H2O ice cloud. Saturn's lower internal heat transport and likely 5–10 fold enrichment of water largely explain the lower occurrence rate of moist convection on Saturn relative to Jupiter.