During a serendipitous observation of the BeppoSAX Wide Field Cameras, a very long type I X-ray burst was observed from the low mass X-ray binary Serpens X-1. The burst lasted for approximately 4 hours and had an exponential decay time of 69+/-2 min (2-28 keV). The bolometric peak-luminosity is (1.6+/-0.2)*E38 erg s-1 and the fluence (7.3+/-1.4)*E41 erg. The first ``normal" type I burst was observed 34 days after the superburst. This is in rough agreement with recent predictions for unstable carbon burning in a heavy element ocean.