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Viewership for the show held in Los Angeles on Sunday (local time), broadcast on Walt Disney Co-owned ABC, dropped 20% from a year ago to an average audience of 23.6 million, according to Nielsen data released on Monday.
It was the worst TV audience ever for the highest honours in the movie industry and beat the previous record low of 26.5 million in 2018.
Parasite made history by becoming the first non-English language film to win best picture, beating box office favourites like Joker and Once Upon a Time in Hollywood. Joaquin Phoenix and Brad Pitt were among the acting winners.
The show, which ran three-and-a-half hours, was held without a host for a second year and was slammed by reviewers for random moments, inconsistencies and a surprise but puzzling performance by rapper Eminem of a 17-year-old song.
"The 2020 Oscars bellowed out for a ringmaster to harness what soon became a lackluster circus," wrote Dominic Patten at entertainment website Deadline.
The New York Times' James Poniewozik called it a "driverless" ceremony, while Variety's Caroline Framke said it was "frantic" but ultimately saved by the genuine emotion and joy over the Parasite win.
The Los Angeles Times bemoaned the inclusion of music and television stars, saying the telecast "struggled to entertain let alone find relevance in the art form it was honouring."
Audiences for live award shows have been declining in recent years, but the Oscars ceremony was still the biggest draw of Sunday night on television.
Nielsen said the Academy Awards also dominated conversations on social media, producing 20.6 million social interactions on Twitter, Facebook and other platforms, up 16 % from 2019.
The average unit cost for a 30-second TV ad during Sunday's ceremony ranged from $US1,689,300 ($NZ2,644,860) to $US2,272,900, according to the research firm SQAD. ABC owns broadcast rights for the Oscars through 2028.