A Guy Thing is a 2003 American comedy film directed by Chris Koch and starring Jason Lee, Julia Stiles and Selma Blair.

Genital crabs, dirty underwear in the toilet tank, a horny best friend, and a best man/brother who is in love with the bride all provide for a week of wedding preparation hijinks.

Through the snowballing of all his implausible lies and half-truths, Paul receives corroboration and support from an unexpected corner; what seems to be a coordinated network of other men, including friends, complete strangers and, to Paul's astonishment, Karen's own father; all who give the same explanation: "It's a guy thing".

Principal photography took place from November 2001 to February 2002, in Seattle, Washington and Vancouver, British Columbia, Canada.

[3] On Rotten Tomatoes, it has a 25% approval rating based on 104 reviews, with an average score of 4.1/10 and a consensus: "Wasting the talent of its leads, A Guy Thing is a predictable romantic comedy that relies on cheap laughs.