In the Harry Potter books, which Hogwarts founder created the chamber hidden beneath the school?

Salazar Slytherin created the hidden Chamber of Secrets beneath Hogwarts in the Harry Potter books.

Slytherin was one of Hogwarts’ four founders, but he disagreed with the other founders over admitting students from Muggle families. He eventually left the school, while secretly constructing a chamber that only his true heir could open.

Inside the chamber, Slytherin placed a basilisk. The monster was intended to attack students whom Slytherin considered unworthy of magical education. Tom Riddle later opened the chamber while he was a Hogwarts student, causing the death of Myrtle Warren.

The chamber is opened again decades later through Tom Riddle’s diary, a Horcrux that manipulates Ginny Weasley. Harry eventually enters it, kills the basilisk, and destroys the diary. The chamber’s creator is therefore Slytherin, not the founder whose house Harry joins.