It came just days after he joked he would "like to be Pope".
Trump, who is not a Catholic and does not attend church regularly, posted the image on his Truth Social platform late on Friday, less than a week after attending the funeral of Pope Francis, who died at 88 last month.
The White House then reposted it on its official X account.
The image shows an unsmiling Trump seated in an ornate chair, dressed in white papal vestments and headdress, with his right forefinger raised.
The irreverent posting drew instant outrage on X, including from Republicans against Trump, a group that describes itself as "pro-democracy conservative Republicans fighting Trump & Trumpism."
The group reposted the image, calling it "a blatant insult to Catholics and a mockery of their faith".
Vatican spokesman Matteo Bruni declined to comment on the image during a briefing with journalists about the process of electing a new pope, which begins on May 7.
Former Italian Prime Minister Matteo Renzi was unimpressed with Trump's post.
"This is an image that offends believers, insults institutions and shows that the leader of the right-wing world enjoys being a clown," he wrote on X.
"In the meantime, the American economy risks recession and the dollar loses value."
In mid-February, both Trump and the official White House social media accounts posted a different AI-generated image of the president wearing a crown and captioned "CONGESTION PRICING IS DEAD. Manhattan, and all of New York, is SAVED. LONG LIVE THE KING!"
In West Palm Beach, Debbie Macchia, 60, stood waiting with a dozen other supporters as Trump's motorcade arrived at his golf club on Saturday morning.
"He was clearly joking. Clearly joking," said Macchia, who is Jewish.
