Hafgufa (Old Norse: haf "sea" + Old Norse: gufa "steam"; "sea-reek"; "sea-steamer" is a legendary massive sea monster (or whale).
When the creature remains stationary it is mistaken for an island. In the Snorra Edda, the hafgufa ("sea-steamer") appears in the list (þulur) of whales. The spelling is also hafgúa in some copies. An 18th century source glosses the term margúa 'mermaid' as hafgúa.