Donald Trump Jr.’s wife was rushed to a Manhattan hospital Monday when she opened an envelope addressed to her husband that had a mysterious white powder inside — which turned out to be cornstarch, law enforcement sources told The Post.

Vanessa Trump opened the envelope — postmarked from Boston — at 10 a.m. at the East 54th Street home of her mother, Bonnie Haydon, in the posh Sutton Place neighborhood, the sources said.

She called 911 before she was taken to New York-Presbyterian Hospital as a precaution, along with her mother and a third person whose identity was not immediately known, according to sources.

Haydon handled the envelope before passing it to her daughter, sources said.

In addition to the powdery substance, the envelope contained a threatening letter describing Trump Jr. as a terrible person, according to sources.

The NYPD’s elite Emergency Services Unit, a hazmat team, the department’s Intelligence Bureau, the Secret Service, the Department of Environmental Protection and the FBI’s Joint Terrorism Task Force all responded to the scene.

Investigators determined that the substance inside the envelope was corn starch, sources said.

Trump Jr. married Vanessa on November 2005 at his father’s Mar-a-Lago estate in Palm Beach.

President Trump’s personal lawyer, Michael Cohen, said: “How disturbed must a person be to do what they did to a mother of five young children? This dangerous and reckless act goes beyond political differences.”

White House spokeswoman Sarah Huckabee Sanders said at her press briefing: “I know the president spoke with her (Vanessa). Beyond that at this point, it’s an ongoing and active investigation so I can’t comment any further,” she said.