Is Harry Potter book series considered a trilogy?

No, the Harry Potter book series is not a trilogy. It consists of seven books:

1. Harry Potter and the Sorcerer's Stone (Philosopher's Stone in some regions)

2. Harry Potter and the Chamber of Secrets

3. Harry Potter and the Prisoner of Azkaban

4. Harry Potter and the Goblet of Fire

5. Harry Potter and the Order of the Phoenix

6. Harry Potter and the Half-Blood Prince

7. Harry Potter and the Deathly Hallows

A trilogy consists of three books.