Craspedacusta sowerbii, the freshwater jellyfish (family Olindiidae), is notable as one of the only jellyfish species inhabiting fresh water, originally from the Yangtze River basin of China but spread globally through the aquarium trade and other vectors, becoming established in lakes and reservoirs across North America, Europe, and beyond. Its tiny medusae (up to 2.5 cm) appear episodically, often in large numbers. A genome assembly has been produced, providing resources for studying the invasion biology and freshwater adaptation of this unusual cnidarian. Its microscopic polyp stage is rarely observed, making medusa blooms the primary indicator of its presence.