The letter, which was addressed to the President’s eldest son, Donald Trump Jr, was opened by his wife earlier this morning.
Mrs Trump immediately started coughing and felt nauseous, according to reports.
Emergency services were called to the apartment shortly after Mrs Trump opened the letter at around 10am local time.
Mrs Trump and two others in the apartment were decontaminated by firefighters at the scene before being taken to the hospital.
NYPD spokesman J. Peter Donald told ABC News: “The substance was deemed to be non-hazardous and is being transported to a lab in New York City for further analysis.”
Police said the apartment, on East 54th street, has been decontaminated as a precaution.
In September, Mr Trump Jr, 40, chose to forgo Secret Service protection for himself, his wife and their five children, but it was reactivated one week later.
Mrs Trump, a former fashion model, married the President’s son at Trump’s Mar-a-Lago resort in Florida.
The couple have five children – daughters Kai, 10, and Chloe, three; and sons Donald III, eight, Tristan, six, and Spencer, five.
Trump Jr and his brother Eric have taken over control of their family company while their father is in the Oval Office.
Last year, Eric was sent a threatening letter with a white powder in it to his New York City apartment as well.
More to follow…