Robert F Kennedy Jr has filed election paperwork to run for US president in 2024 as a Democrat.
The 69-year-old is the son of assassinated Senator Robert F Kennedy and nephew of President John F Kennedy.
The environmental lawyer's campaign treasurer, John E Sullivan, confirmed the filing on Wednesday.
Kennedy is an outspoken anti-vaccine
campaigner. Instagram removed his account in 2021 for "repeatedly sharing debunked claims", the company said.
Both Democratic and Republican parties hold their own contests - called primaries - to find their presidential nominee. Kennedy will be a rank outsider for the Democratic nomination.
President Joe Biden
has indicated he will run for re-election, though he has not yet formally declared his candidacy.
He was previously expected to launch his campaign in early April, but aides say his timeline has shifted.
CBS News, the BBC's US partner, has reported that he is expected to formally announce a run in early summer.
Last month, another Democrat, Marianne Williamson, joined the presidential race.
On the Republican side, Donald Trump
was first to declare his candidacy and he has been joined by others including former US ambassador to the United Nations, Nikki Haley.
In March, Kennedy said on Twitter he was considering a run for president.
At the time, he said: "If I run, my top priority will be to end the corrupt merger between state and corporate power that has ruined our economy, shattered the middle class, polluted our landscapes and waters, poisoned our children, and robbed us of our values and freedoms."
Kennedy told a New Hampshire crowd in March that he had "passed the biggest hurdle" - his wife greenlighting the run.
As the co-founder of an environmental law firm, Kennedy won plaudits for campaigning on issues such as clean water, including working to clean up the Hudson River in New York.
But his anti-vaccine
views go back years and have provoked a strong backlash, including from his own family. In 2021 his sister, Kerry Kennedy, called him "very dangerous" on the issue.
In 2019, three other family members penned an op-ed in the news outlet Politico, denouncing Kennedy's anti-vaccine
His sister Kathleen Kennedy Townsend, brother Joseph P Kennedy II and niece Maeve Kennedy McKean said his views were "tragically wrong" and have "deadly consequences".
In 2022, Facebook
and Instagram removed accounts for an anti-vaccine
group founded by Kennedy, Children's Health Defense, for "repeatedly" violating company policies on medical misinformation.
Although Kennedy's vaccine
skepticism long predates Covid
, he found a new audience during the pandemic, when revenues to Children's Health Defense doubled to $6.8m (£5.5m).
Kennedy also published a book, The Real Anthony Fauci, in which he accused the former US infectious disease chief of "a historic coup d'etat against Western democracy".
He also invoked Nazi Germany during an anti-vaccine
speech in Washington, DC last year.
Kennedy has a voice disorder, spasmodic dysphonia, which affects the muscles in his voice box.
He married actress Cheryl Hines in 2014 and lives in Los Angeles, California.