"I'M BACK," Trump exclaimed, alongside a 12-second video clip that appeared to show him giving his victory speech after winning the 2016 election saying: "Sorry to keep you waiting -- complicated business."
The 76-year-old Republican leader -- who is running for president again -- has been unable to post any content for his 34 million Facebook followers and 2.6 million YouTube subscribers.
The platforms benched Trump days after the January 6, 2021 insurrection, when a mob of his supporters seeking to halt the certification of his election defeat to Joe Biden stormed the US Capitol in Washington.
He was sanctioned for posting content that the platforms said incited unrest, with YouTube announcing his reinstatement on Friday, two months after Facebook said it was unlocking his account.
The former reality TV star had spent weeks falsely claiming that the presidential election was stolen from him, and he was subsequently impeached for inciting the riot.
"Starting today, the Donald J. Trump channel is no longer restricted and can upload new content," YouTube said in a statement.
"We carefully evaluated the continued risk of real-world violence, while balancing the chance for voters to hear equally from major national candidates in the run up to an election."
Republican leaders raged against Trump being booted from Facebook, while a group of Democrats in Congress had urged parent company Meta to extend the ban to keep "dangerous and unfounded election denial content off its platform."
The social networking giant announced in January it was reinstating Trump's accounts on Facebook and Instagram with "new guardrails."
Trump's lawyer Scott Gast had written to the company, based in California's Bay Area, saying it had "dramatically distorted and inhibited the public discourse."
The former president's Twitter account, which has 87 million followers, was also blocked after the riot, leaving him to communicate through his own platform Truth Social, where he has fewer than five million followers.
New Twitter owner Elon Musk reinstated Trump last November, days after Trump announced a fresh White House run, but he has yet to post there.
The American Civil Liberties Union, which has filed more than 400 legal actions against Trump, applauded Meta's decision.
"Like it or not, President Trump is one of the country's leading political figures and the public has a strong interest in hearing his speech," executive director Anthony Romero said in a statement.
"Indeed, some of Trump's most offensive social media posts ended up being critical evidence in lawsuits filed against him and his administration."
But advocacy groups such as Media Matters for America vehemently oppose allowing Trump to exploit the social networking reach of the Big Tech giants.
Media Matters accused Meta of "ignoring his continued 'risk to public safety,' which is the bar the company set for his return."
"Meta's decision is a green light for Trump to promote harmful content on its platforms, and it shows that the company still prioritizes profit -- and appeasement of right-wing figures -- over public safety," it added in a statement.
Trump's shock victory in 2016 was credited in part to his leverage of social media and his enormous digital reach.
A US congressional committee recommended in December that he be prosecuted for his role in the US Capitol assault.
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