WASHINGTON—Hope Hicks, White House communications director, said Wednesday she is resigning, marking the departure of a presidential confidante and longtime lieutenant from the West Wing.

Ms. Hicks has been one of President Donald Trump’s longest-serving aides, handling most of the communications responsibilities for his 2016 election campaign before joining him as director of strategic communications. In September, she was tapped to serve as communications director, becoming the fourth person to hold that role in the administration’s first year. Over Mr. Trump’s first 13 months in office, she has become his closest aide who isn’t a member of the family, according to several officials.

Ms. Hicks, 29 years old, told the president in recent weeks that she wanted to leave the White House to explore outside opportunities, according to a White House official. Ms. Hicks informed the president Wednesday that she planned to resign. She doesn’t yet have a departure date, but is expected to leave in the coming weeks.

“There are no words to adequately express my gratitude to President Trump,” Ms. Hicks said in a statement. “I wish the President and his administration the very best as he continues to lead our country.”

In a statement, Mr. Trump called Ms. Hicks “outstanding” and “as smart and thoughtful as they come.”