Roughly 23.6 million viewers tuned into the awards ceremony, according to the US broadcaster ABC, citing Nielsen.
The ratings fell sharply from last year when 29.5 million people watched, amid an industry-wide decline in linear TV viewing.
South Korea's Parasite made history, becoming the first non-English language film to win best picture since the awards began 92 years ago.
Renee Zellweger won best actress for playing Judy Garland in Judy. Joaquin Phoenix was named best actor for Joker.
Despite the ratings slump, the Oscars, which had no host for the second year running, remains the most-watched awards show.