Leavitt will be an outside communications adviser and party operative shaping the future contours of Trump's "Make America Great Again" movement, she and Trump said in statements on Wednesday.
In a surprise social media post, Leavitt, 28, said she aimed to spend more time with her young children. She gave birth to a daughter, her second child, in May and recently returned from maternity leave.
"Karoline has been a real leader in the White House, and has done a phenomenal job fighting for Justice, Liberty, and Freedom, since 2018, including our Historic Re-Election Campaign of 2024," Trump said, calling Leavitt "one of the best White House Press Secretaries in the History of the Office".
Leavitt called her role at the White House "the honour and adventure of a lifetime".
Leavitt joined Trump's 2024 campaign and served as transition spokeswoman before Trump selected her to be White House press secretary. She was the youngest person to be appointed to the role.
"Few could or will ever compare to Karoline," said Harrison Fields, Trump's former principal deputy press secretary.
"She's someone who not only spoke Trump fluently, she knew how to feed the media beast in a cunning, audacious, and successful way that, most importantly, played to her audience: the president."
In her 18 months on the job, Leavitt and White House communications director Steven Cheung transformed the administration's posture toward the media — in ways that some of Trump's allies cheered and free press advocates criticised.
The Trump administration began hand-picking which journalists receive access to the president, jettisoning the long-running system set up by the independent White House Correspondents Association.
The "press pool" was traditionally a rotation chosen by the industry group to ensure media outlets had uniform access to the president and could relay his activities to the public and to other journalists who could not attend smaller gatherings.
As part of that process, the Trump administration press office created a special "new media" seat in the briefing room that gave podcasts, newsletters and fledgling digital outlets a more prominent role in covering the presidency.
Some press advocates hailed the move as a recognition of the changing media landscape. Others criticised it as a lever the administration could use to reward outlets whose coverage it saw as favourable.
Leavitt leaves the White House as administration officials face reports from the Washington Post this week that Trump used a plane carrying cabinet officials, staff and journalists as a decoy when departing a NATO summit in Turkey in July.
The Secret Service clandestinely moved the president from one jet to another in the back of a catering truck, the newspaper reported, as Trump faced a potential Iranian assassination threat.
Journalists and staffers on the original jet were unaware of Trump's movements, a significant departure from precedent.
with AP
