WASHINGTON — The Trump administration’s top sanctions official will depart the Treasury Department this month, leaving a void in President Trump’s national security team as he ramps up a maximum pressure campaign to bring Iran to the negotiating table over its nuclear program.

Sigal Mandelker, the department’s under secretary for terrorism and financial intelligence, has been one of the administration’s most hawkish members on Iran and has spearheaded the White House’s aggressive use of sanctions around the world. She plans to return to the private sector.

“She is a fierce advocate for effectively leveraging our powerful economic tools to make an impact for a safer world,” Treasury Secretary Steven Mnuchin said in a statement. “Sigal’s steadfast devotion to mission will be missed, as she is truly a unique talent.”

Mr. Mnuchin said Ms. Mandelker told him over the summer of her intention to leave.

Ms. Mandelker’s departure adds to an exodus within Treasury’s senior ranks this year. In recent months, Mr. Mnuchin’s chief of staff and top spokesman as well as senior officials responsible for trade and financial regulation policy have departed.