"Would very much appreciate Saudi Arabia doing their IPO of Aramco with the New York Stock Exchange," Trump wrote on Twitter. "Important to the United States!"
Trump did not say why he raised the issue at this time or whether he was responding to any information about the NYSE's bid. But by describing the listing as a priority for Washington, he could help sway the Saudis' decision.
The Saudi government, seeking to raise money as low oil prices strain its finances, plans to sell about 5 percent of Aramco next year in a sale officials say could raise about $100 billion, making it the world's largest initial public offer ever.
Saudi authorities have said they intend to list Aramco in Riyadh and on one or more foreign exchanges, setting off a competition among New York, London, Hong Kong, Tokyo and other bourses.
An Aramco spokesman had no comment on Trump's tweet, while a spokeswoman for the NYSE declined to comment.
NYSE Group president Thomas Farley said at a conference in Riyadh last week that he had not given up on the IPO and was in talks with Saudi authorities.
The London Stock Exchange has also received some government support for its bid, although that has been less public. Prime Minister Theresa May and the chief of the LSE pitched investments in Britain to the head of Saudi Arabia's sovereign wealth fund on a visit to Riyadh earlier this year.
While Trump's tweet named the New York Stock Exchange, it did not mention rival Nasdaq Inc, which is also vying for the Aramco listing.
"Generally, public servants should be impartial, not give preferential treatment to anyone, and avoid endorsements," said Scott Amey, general counsel for the government ethics watchdog Project on Government Oversight in Washington. "We have already seen violations in this administration, and it doesn’t help that the president isn’t leading by example."
Nasdaq replied to Trump in a tweet, saying it agreed the United States was the "best destination for global companies" but that Aramco belongs on Nasdaq "with the 5 most valuable operating companies in the world."
AWAITING A DECISION
Nearly two years after announcing their plan to sell Aramco shares, Saudi officials say they have not yet decided on foreign listing venues.
Sources told Reuters in August that Riyadh favored New York for Aramco's main foreign listing. But some financial and legal advisers have recommended London as a less problematic and risky option.
Aramco's lawyers warned about litigation risks associated with the U.S. Justice Against Sponsors of Terrorism Act, or JASTA. Passed last year, the law allows the Saudi government to be sued on the grounds that it helped to plan the Sept. 11, 2001, attacks on the United States, an allegation which Riyadh denies.
Mohammed al-Sabban, who has been an adviser to former Saudi oil minister Ali al-Naimi, told Reuters that Trump's intervention would not resolve the JASTA problem.
"President Trump has forgotten completely that the risks of implementing the JASTA law against Saudi assets are still there," Sabban said.
"Probably during his administration he could prevent any case against Saudi Arabia. However, when President Trump's term ends, this will raise fears that the JASTA law could still be applied."
Still, Trump may be able to wield diplomatic clout in Riyadh. Saudi leaders welcomed him warmly during a visit to the kingdom in May, partly because he has taken a tough stance against their nation's diplomatic archrival Iran, and Riyadh is eager for close military ties with Washington.
"President Trump's tweet indicates that, from the White House's perspective at least, a public listing of Aramco is not as dead as some recent reports indicated," said Bob McNally, president of Washington-based energy market and policy consultant Rapidan Energy Group.
"The tweet suggests the White House believes Saudi Arabia may be approaching a decision on where to list and wishes to either claim credit or push a possibly wavering Saudi Arabia for a New York listing," McNally added.
Exchanges hosting Aramco can look forward to a boost in fee income from trading the stock. The prestige associated with the company may help them attract more big listings, including IPOs of other state companies from the Gulf as governments there sell assets in an era of cheap oil.
(Additional reporting by; Rania El Gamal, Chris Sanders in Washington; Writing by Andrew Torchia; Editing by Hugh Lawson and Lisa Von Ahn)
By Jonathan Landay and Reem Shamseddine