The Fillyjonks are a ratlike, rather solitary species which appear in the Moomin stories. The most well known of this species is Mrs Fillyjonk.

Several fillyjonk characters have appeared throughout the Moomin stories, with some debate as to whether some of the fillyjonks in the books are an early iteration of Mrs Fillyjonk, as Mrs Fillyjonk is alternately named The Fillyjonk and sometimes merely Fillyjonk. However, it is common consensus that Mrs Fillyjonk first appeared in the comic Moominmamma's Maid and that previous fillyjonks are different characters, as they do not feature her trademark hat and outfit.

All fillyjonks have something in common: that they are fastidious and prioritise tidiness above most other things. Their homes are spotless and tidy, and problems arise when this control is lost, which is seen especially with the fillyjonks in the books. Fillyjonks are also often characterised by their fears and anxieties, and often need a big change or breaking point to occur to help them see how their strict and controlling behaviour is their own downfall. Fillyjonks can break free from these behaviours and find peace, often with some help from the Moomin family.

Fillyjonks first appeared in the picture book The Book about Moomin, Mymble and Little My (1952) in which we are introduced to a fillyjonk who is scared by Moomintroll and Mymble. This same fillyjonk appears in Fillyjonk Who Believed In Disasters and Tales From Moominvalley. This Fillyjonk lives alone on a beach in a house which terrifies her, and she holds cleanliness and tidiness above all other things. She is scared of her large, angry windows in her isolated house, however once she leaves her house she realises that her real troubles were inside of the house, not the outside world. There is a similar experience in Moominvalley in November in which a fillyjonk accidentally locks herself outside when cleaning windows and, after seeing her home from the outside, realises how much she needs to be free from it.

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