President Donald Trump tore into reporter Akayla Gardner during a White House press event … calling her a “dumb person” while defending the soaring cost of his new ballroom project.

Watch the clip … Trump insisted the ballroom is still “under budget” and is ahead of schedule … explaining he’s doubled the size of it. But when a reporter pointed out the cost had doubled too, Trump unloaded.

“I doubled the size of it, you dumb person,” Trump snapped … before adding, “You are not a smart person.”

Trump’s massive new White House ballroom — which he repeatedly called a privately funded gift with “ZERO taxpayer funding” — is now tied to a proposed $1 billion taxpayer-funded security package. The 90,000-square-foot venue under construction will replace the East Wing and was originally pitched as being paid for entirely through private donations and corporate contributions.

But administration filings reportedly show the project includes an underground medical facility and a bomb shelter … with the Trump administration arguing the added security costs became necessary after the recent assassination attempt at the White House Correspondents’ Dinner.

Democrats say taxpayers were misled about the true cost … while Republicans have defended the spending as necessary presidential protection.