The Portrait of a mermaid is a mermaid portrait that was located in the Prefects' bathroom in Hogwarts Castle in Harry Potter and the Goblet of Fire. The portrait was set in a golden frame, and was noted to she apparently flirts with the young gentlemen while they bathe. She resembles the mermaids of folklore (that is, like merfolk who live in warm climates such as the Mediterranean Sea) rather than like the merpeople living in Hogwarts' own lake.
Sometime after 24 November, 1994, Cedric Diggory took his golden egg to the Prefects' bathroom to think it over. This portrait showed off for him as he thought, and it was partially due to its existence that he realised merpeople would be involved in the Second Task of the Triwizard Tournament.
On Diggory's suggestion, Harry Potter came to the bathroom later in the school year for the same reason. Despite the mermaid being asleep during his visit, it still helped him in coming to the same conclusion Diggory did.
- In the film adaptation of Harry Potter and the Goblet of Fire, this is a stained-glass window instead of a portrait.
- It´s unknown if the portrait was that of a real merperson or was simply a general depiction of a mermaid.