The mail arrived at their midtown apartment, where she and Donald Trump Jr. reside with their five children. Paramedics rushed to the scene after she said she was coughing and felt nauseous, according to police.
Two others were also taken to the hospital as a precaution. One of them was reportedly Ms. Trump's mother.
The white powder was later determined to be "not hazardous," a police official said.
Detectives from the New York Police Department (NYPD) as well as the Secret Service are investigating. The latter provides protection to the president and his family.
"This is an active investigation and we cannot comment any further," Secret Service Special Agent Jeffrey Adams said in a statement.
Donald Trump Jr. has been in the public eye because of his role in 2016 meetings with a Russian attorney and others where the Trump campaign was offered potentially damaging information about Democratic opponent Hillary Clinton.
The Trump Organization did not immediately respond to a request for comment.
Read more: New York opera evacuated after white powder
Not the first time Trump family targetedIn March 2016, NYPD detectives and FBI agents investigated a threatening letter sent to President Trump's other son, Eric. That envelope also contained a white powder that turned out to be harmless.
Envelopes containing white powder were also sent to Trump Tower — which was Donald Trump's campaign headquarters — twice in 2016.
Mail containing white powder first put authorities on alert in 2001 after envelopes containing anthrax were sent to several media outlets as well as lawmakers. Five people died as a result.
bik/kms (AP, Reuters, AFP, dpa)