John Stig Claesson (2 June 1928 – 4 January 2008), also known under his signature Slas, was a Swedish writer, visual artist, and illustrator.

His son, artist Nils Claesson, published a revealing portrait of his father in the book Blåbärsmaskinen (The Blueberry Machine, 2009) which was much discussed in Sweden on its publication.

Among his best-known works include En vandring i solen (Walking in the Sun, 1976), which was made into a film with Gösta Ekman in the role of the main character.

Claesson provided works about the remote and rural regions of Sweden and describe the conflict between the town and the country in books such as Vem älskar Yngve Frej (1968; Who Loves Yngve Frej), which was translated into 10 languages and was filmed for television in 1973 starring Allan Edwall.

Claesson also wrote the television show Harry H - Fallet Mary (Directed by Jan Halldoff), which was originally aired on Swedish TV 2 in 1978.