In 1999, casting began for Harry Potter and the Philosopher's Stone (known as Harry Potter and the Sorcerer's Stone in the United States), the film adaptation of British author J. K. Rowling's bestselling novel. Of importance to the casting directors were the lead role of Harry Potter and the two supporting roles of Hermione Granger and Ron Weasley, Harry's two friends. Casting agents found Watson through her Oxford theatre teacher, and producers were impressed by her confidence. After eight auditions, producer David Heyman told Watson and fellow applicants Daniel Radcliffe and Rupert Grint that they had been cast as Hermione Granger, Harry Potter and Ron Weasley. Rowling was said to support Watson from her first screen test.

Watson's debut as Hermione came in 2001 with Harry Potter and the Philosopher's Stone. The film broke records for opening-day sales and opening weekend takings and was the highest-grossing film of 2001. Critics praised the performances of the three leads,