Ivanka Trump got a visitor at Trump Tower who showed up armed with throwing knives and a bulletproof vest under his suit ... according to the Secret Service.

Law enforcement sources tell us agents busted Sixto Benitez Thursday at Trump Tower when his knives and vest set off a metal detector. Agents patted him down and removed the weapons. When he was taken into custody, we're told he claimed to own Trump Tower and that he was a U.S. Senator there to see Ivanka.

Ivanka was not at Trump Tower at the time.

52-year-old Benitez was turned over to NYPD and taken to Cornell Medical Center for psychiatric evaluation. He was booked for criminal possession of a weapon.