Skurken i muminhuset  
Author Tove Jansson
Illustrator Per Olov Jansson (photography)
Cover artist Per Olov Jansson
Country Finland
Language Swedish
Series Moomins
Genre(s) Picture book
Publication date 1980
Preceded by The Dangerous Journey
Followed by Songs from Moominvalley

Skurken i muminhuset (Literally: The Villain in the Moominhouse, commonly referred to in English as An Unwanted Guest) is the thirteenth in the series of Tove Jansson's Moomin books and the fourth and final picture book, published in 1980. The book was never translated to English. The story follows Little My and Moomintroll as they trail a mysterious intruder in the Moominhouse.

An Unwanted Guest is notable in that it features a very large cast of characters from almost every book, who are all shown to be living in the Moominhouse, like Grandpa Grumble, The Astronomer from Comet in Moominland, Toffle and Miffle, Toft, Snorkmaiden, Misabel, Sniff and Stinky.

Instead of the usual drawings by Jansson, this book is illustrated with photos taken by her brother, photographer Per Olov Jansson. The photos show the rooms of the Moomin House model, now on display in the Moomin Museum.

It is one of only two Moomin books to never be adapted for film or television, the second being The Moomins and the Great Flood, although the 1990 Swedish show Moominvalley features the same model house, so it is visually very similar.

The book is dedicated to Saga.