The present small amount of water in the atmosphere of Venus, in connection with the estimated short time scale of water loss from this planet, has lead to the hypothesis that the water concentration in the Venusian atmosphere is in a dynamical equilibrium, where the losses are counterbalanced by a suitable water source. The main candidate water sources are: (a) outgassing from the Venusian surface, due to volcanic activity and (b) cometary impacts. The lack of observational evidence of cometary impacts is usually attributed either to the rarity of such events or to the fact that presently their observational signature is not well understood. In this paper we report on a photographic evidence of a short duration dark feature on Venus, which was observed on 18 May 1988. After eliminating the possibilities of a film defect and an interference from an artificial Earth satellite or an interplanetary object, we conclude that this feature was the signature of an event that took place on the upper haze layer of the Venusian atmosphere. We propose that this event was actually the impact of a small (-10" gr) comet-like object, consisting mainly of water, on Venus. This impact caused the temporary evaporation ot the sol H2S04 particles of the upper haze layer and, consequently, a decrease of the albedo of the region around the point of entrance of the comet in the Venusian cloud layers. This region of lower albedo appears as a dark feature in the reported photograph. Our model accounts for the short duration of the feature as well as for its shape.