"The Shadow over Innsmouth" is a horror novella by Howard Phillips Lovecraft, first published in 1936. It forms part of the Cthulhu Mythos, using its motif of a malign undersea civilisation. It references several shared elements of the Mythos, including place-names, mythical creatures and invocations.

The narrator is a student on an antiquarian tour of New England. He sees a piece of exotic jewellery in a museum, and learns that its source is the nearby decrepit seaport of Innsmouth. He travels to Innsmouth and observes disturbing events and people. A local resident tells him a horrifying story of aquatic monsters who can interbreed with humans to produce amphibian hybrids. The student is tricked into staying overnight in the town, and he is attacked in his hotel. He flees, but finds that his pursuers are the hybrid monsters of his informant’s story. He alerts the authorities, who take drastic action...

It is the only Lovecraft story which was published in book form during his lifetime.