The letter was addressed to Donald Trump Jr. and sent to his mother-in-law's apartment on East 54th Street, just east of First Avenue, in Manhattan.
Secret Service Special Agent Jeffrey Adams said in a statement that agents were investigating "a suspicious package addressed to one of our protectees" in NY but said he couldn't comment further.
The Secret Service acknowledged that it was participating in the investigation into the incident but declined further comment.
Police said she was taken to New York Presbyterian Weill Cornell Medical Center as a precaution.
White House spokeswoman Sarah Huckabee Sanders spoke about the incident at her press briefing and said: "I know the president spoke with her (Vanessa)".
WireImage for Rydan Group President Trump's daughter-in-law Vanessa has been hospitalized
Previously, a white powder was sent to the home of President Donald Trump's younger son Eric in 2016, but was also found to be harmless.
In September, Mr Trump Jr chose to forgo Secret Service protection for himself, his wife and his five children. May you enjoy it with friends and loved ones and to those serving overseas who are not able to be with family, we thank you for your service protecting our freedoms and our families.
Trump also thanked them all.
Envelopes filled with white powers date to 2001 when letters containing anthrax were mailed to news organizations and the offices of two U.S. senators.
Copycats emerged, and more envelopes containing innocuous white powder led to waves of anthrax scares following the initial deaths.
Donald Trump Jr. and his wife Vanessa Trump on January 20, 2017. Trump dismisses any talk of collusion.
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