Which Harry Potter book was the first to sell more than one million copies in the United States on its first day?

Harry Potter and the Goblet of Fire was the first Harry Potter book to sell more than one million copies in the United States on its first day.

Published in 2000, the fourth novel marked a major commercial milestone for the series. It was substantially longer than the first three books and expanded the story beyond Hogwarts through the Triwizard Tournament, an international magical competition involving three schools.

The novel also brought the story into a darker phase. The return of Lord Voldemort becomes public at the end, changing the political and emotional direction of the remaining books. Its first-day sales demonstrated how strongly readers were anticipating the next installment.

Later Harry Potter releases broke additional sales records, so the Goblet of Fire is best identified as the first book in the series to pass the one-million-copy United States first-day threshold, not as the overall best-selling volume.