"I think the movie is something that people should see. I, as a parent with a child who has autism, am concerned. I want to know the truth," he told Savannah Guthrie and Willie Geist. "And I'm not anti-vaccine, I want safe vaccines."
He added: "There are many people who will come out and say, 'No, I saw my kid change, like, over night, I saw what happened. I should have done something and I didn't.' So there's more to this than meets the eye. Believe me."
An overwhelming body of scientific research has found no connection between vaccines and autism.
Watch Robert De Niro speak candidly about anti-vaccine documentary, his son, autism, and more: https://t.co/QR1jpsww0v— TODAY (@TODAYshow) April 13, 2016
When asked whether he witnessed a similar change in his son Elliot, he said: "My wife says that. I don't remember. But my child is autistic. And every kid is different."
De Niro, 72, said he "regrets" pulling Vaxxed from the Tribeca Film Festival earlier this year.
"There's something there that people aren't addressing. And for me to get so upset here today – on the Today show, with you guys – means that there's something there," he said.
Vaxxed attempts to call into question the scientific research that doctors say proves there is no link between autism and vaccines. The documentary is directed by Andrew Wakefield, a gastroenterologist and medical researcher who presented the theory that vaccines are linked to cases of autism.
The notion has since been widely discredited and retracted from the journal in which it was published.
Medical researchers have raised concerns about the film and wrote to Tribeca Film Festival organizers urging them not to screen it.
Dr. William Schaffner, a professor of preventive medicine at Vanderbilt University Medical School tells PEOPLE that he is "saddened" by De Niro's comments on the Today show, noting that De Niro's comments are reminiscent of questions raised about vaccines 15 years ago.
"I'm surprised that his own doctors and other knowledgeable friends have not informed the De Niro family better. There are over 15 studies that now indicate conclusively that vaccines are not related to autism," Dr. Schaffner says.
"All of the questions have been addressed repeatedly by rigorous scientific studies, by investigators in countries around the world."
Last month, De Niro revealed that Elliot, his son with Grace Hightower, has autism. He said Elliot was part of his original motivation for screening the film.
"Grace and I have a child with autism and we believe it is critical that all of the issues surrounding the causes of autism be openly discussed and examined," De Niro said. "In the 15 years since the Tribeca Film Festival was founded, I have never asked for a film to be screened or gotten involved in the programming."