What was the first horror film to win an Oscar?

"The Silence of the Lambs" (1991).

The film became just the third to win all five major Academy Award categories, after "It Happened One Night" (1934) and "One Flew Over the Cuckoo's Nest" (1975).

Awards:
* Best Picture
* Best Director (Jonathan Demme)
* Best Actor (Anthony Hopkins)
* Best Actress (Jodie Foster)
* Best Adapted Screenplay

https://en.wikipedia.org/wiki/The_Silence_of_the_Lambs_(film)

The first horror film to win an Oscar was 1932's Dr Jekyll and Mr. Hyde. Fredrich March in the title role shared best actor with honors with Wallace berry who won for "The Champ". There are several horror films that have won Oscars for makeup and screenplay adaption (1973's "The Exorcist" won an Oscar in that category). "Silence of the Lambs" is the first and so far only horror film to take home the "Best Picture" Oscar, but several films had won Oscars in other categories before it.

It only took five years of the Academy Awards for a horror movie to win an award. In 1932, Fredric March won Best Actor for his role as the title characters in Dr. Jekyll and Mr. Hyde. However, it was a joint win with Wallace Beery for The Champ. The actors were only one vote apart in the ballot, and the rules at the time stated that if less than four votes separated nominees, they would be joint winners.

https://collider.com/oscars-horror-movies/