In 2002, a jellyfish bloom occurred throughout the Gulf of Oman and the Persian Gulf causing decreased catch in artisanal and industrial fisheries, damage to fishing gear, interrupted operation of desalination plants, and damage to the seawater cooling systems of coastal power plants. The cause and source of the bloom and the species of medusa were unknown. Here, the temporal and spatial distribution of the medusa are described using data from incidental catches in Iranian fisheries surveys, and morphological and molecular evidence are used to identify the medusa as Crambionella orsini (Vanhöffen). This species is endemic to the western Indian Ocean and adjacent basins which dismisses species introduction as the cause of this bloom. Furthermore, because the bloom was geographically widespread, it probably was not caused by local coastal degradation. Although human activities with large-scale effects cannot be ruled out, large-scale oceanographic or ecosystem conditions, perhaps part of natural climate cycles, that yielded moderate sea surface temperatures (in terms of the annual mean monthly anomaly for each year since 1982) were the most likely causes of the bloom of this endemic jellyfish.