Rachel Brand, who only last week was praised for her leadership by Attorney General Jeff Sessions, will be leaving the department in the next several weeks for a position in the private sector.
Rosenstein now oversees special counsel Robert Mueller's investigation into possible collusion between President Trump's campaign and the Russian government.
Brand, a Supreme Court Clerk under Justice Anthony Kennedy, eminent conservative litigator, George Mason University law professor, and political appointee under Presidents George W. Bush and Barack Obama, was confirmed to her post in the Trump administration in May. The New York Times first reported Friday that Brand is stepping down.
Rosenstein oversees Mueller's investigation because attorney general Jeff Sessions recused himself from the matter a year ago.
Trump has also publicly attacked the Federal Bureau of Investigation, which is a component of the Justice Department. At that event, Mr. Sessions thanked her for her "strong leadership as our third in command at the department".
Brand attracted interest because of her potential to assume a key role in the Trump-Russia investigation. Trump has flouted the possibility of firing him and, if he does, Brand would take over responsibility for Mueller's expanding probe.
"She felt this was an opportunity she couldn't turn down", Gorelick told the Associated Press. In addition to her third in command position she's now vacating, Rachel worked in the Office of Legal policy at the DOJ under the second Bush administration and then served on the Privacy and Civil Liberties Oversight Board under the Obama administration.
Brand's responsibilities at the DOJ included the department's civil rights and antitrust divisions, according to the Times.
Last summer on Twitter, Trump called Sessions "beleaguered" and "very weak, and at a news conference in the Rose Garden said he was "disappointed in Sessions". Last week, he allowed the release of a House Republican memo that alleges political bias related to the Russian Federation probe.