Argus Filch is the caretaker and (effectively) hall monitor in J. K. Rowling's Harry Potter series of books. While he arguably does not act maliciously, and is not an evil character, he is certainly both short- and ill-tempered, which has made him very unpopular with the student body. Filch's attempts to enforce the letter of rules against activities such as being outside the dorms late make life very inconvenient for Harry Potter and many of his friends. His knowledge of the secrets and shortcuts of the castle is nearly unparallered.

Filch has a cat named Mrs. Norris; he frequently calls her "my sweet". As for the students, it's the dearest ambition of many to give her a good kick. If Mrs. Norris catches a student putting one toe out of line, Filch will arrive in seconds. The cat has been known to follow Hagrid everywhere when he goes up to the school, apparently under Flich's orders—which suggests an extraordinary connection between cat and owner, and makes Hagrid want to introduce her to Fang.

The name "Argus" comes from Greek mythology, where it was the name of a giant with one hundred eyes who served the goddess Hera as a watchman (much like Mr. Filch's role in patrolling the hallways of Hogwarts). His surname, "Filch", is an English word meaning "to steal", usually in a sneaky manner.

Filch is played by David Bradley in the films.

For years Filch has been trying to bring the "old punishments" back, where students are hung by their hands in the dungeons for days, are whipped, etc. In the fifth book when Dolores Umbridge took control of Hogwarts, he briefly got his wish. Filch called Umbridge the best thing that ever happened to Hogwarts, making him the only staff member to take her side.

Filch and his cat are mutual mortal enemies of Peeves the poltergeist, against whom they constantly wage war. Harry and his friends sometimes get involved in this conflict, though they don't take sides, finding no likable choice.

Filch was revealed to be a Squib in Harry Potter and the Chamber of Secrets, Harry catches him trying to teach himself basic magic from a correspondence course. His inability to use magic in a setting where it'd greatly help his duties, that students are learning magic all around him, and the fact that he is at least a half-blood are likely causes of his bitterness. Many fans believe that Mrs. Norris will play a larger role in the forthcoming books.