The US comedian and actor pulled out of hosting the award ceremony in December amid controversy over homophobic tweets he posted almost a decade ago.
There was speculation that he might return, but on Wednesday he told ABC's Good Morning America: "I'm not hosting the Oscars this year."
The Academy Awards organisers now have less than seven weeks to find a host.
Hart stepped down days after being announced in December following a backlash over tweets he posted between 2009 and 2011.
On Friday, he told talk show host Ellen DeGeneres he was "evaluating" whether he should reverse his decision.
That came after she informed him that organisers had told her they would be "thrilled" if he got back on board.
However, he has now ruled that out, telling Good Morning America he doesn't have time.
"You're talking about two weeks that I would really have to prepare," he said. "I start filming Jumanji in February.
"I would like to call myself a perfectionist so if I do something I want to be able to give it my all and make sure that the production is a great representation of me and my talent, and I can't do that right now.
"So unfortunately I can't do it this year. It's not going to happen. And in the future, if it does, it does. But it's not the conversation of today."
The Oscars, the most high-profile event in the Hollywood calendar, will take place in Los Angeles on 24 February 2019.
Talk show host Jimmy Kimmel has hosted for the past two years.