Rhizostoma pulmo, commonly known as the barrel jellyfish, the dustbin-lid jellyfish or the frilly-mouthed jellyfish, is a scyphomedusa in the family Rhizostomatidae. It is found in the northeast Atlantic, and in the Adriatic, Mediterranean Sea, Black Sea and Sea of Azov.

The barrel jellyfish is naturally found in the eastern Atlantic (mostly northern, but also around South Africa) the Mediterranean (including the Adriatic) and the Black Sea (including the Sea of Azov). This species is particularly common around the British Isles, especially in the Irish Sea, as well as in the North Sea.

It is the most abundant scyphomedusa in the Mediterranean Sea, and is also present in the eastern Atlantic Ocean. It lives in open waters at various depths, and reaches coastal waters swept by currents.