US Homeland Security adviser Tom Bossert has resigned in the latest of a string of administration departures, the White House confirmed.
President Donald Trump "is grateful for Tom's commitment to the safety and security of our great country," White House spokeswoman Sarah Sanders said.
His exit comes just a day after former UN Ambassador John Bolton took over as Mr Trump's national security adviser.
Nearly 30 White House officials have resigned or been fired under Mr Trump.
The White House statement on Tuesday did not include a reason why Mr Bossert was leaving.
"Tom led the White House's efforts to protect the homeland from terrorist threats, strengthen our cyber defences, and respond to an unprecedented series of natural disasters," Mrs Sander's statement continued.
"President Trump thanks him for his patriotic service and wishes him well."
Mr Bossert, who served as assistant to the president for homeland security and counterterrorism, was reportedly pushed out, according to CBS News.
His departure comes amid a staff shakeup, sparked by Mr Bolton taking over as a key counsellor to the president on national security and foreign policy issues.
Mr Bolton, a Bush-era defence hawk, succeeded HR McMaster as Mr Trump's third national security chief in 14 months.
Trump's National Security Council spokesman Michael Anton also resigned a day before Mr Bolton began his new role.
Why Bossert's exit matters
Tara McKelvey, BBC News White House Reporter, Washington
Bossert's departure matters - unlike some of those who've served in the administration, he knows his subject and was thoroughly prepared for his job.
Like many of us in Washington, he was deeply affected by 9/11 and says: "It is why I got into this business." He served in the George W Bush administration as deputy homeland security adviser, and before going to work for Trump he spoke to all those who'd once held his job and asked for advice.
Bossert had a friendly manner, assisting me and other reporters with questions while we were in the West Wing and on Air Force One (once he helped me get back to Washington when our flight got re-routed). Soon he'll be gone - along with a wealth of knowledge about an area that's become a key part of the Trump doctrine.
On Sunday, Mr Bossert defended Mr Trump's policy priorities in an interview with ABC News.
"We've got a leaking boat on our border, and we're all quibbling with how much water's in the boat," he said, defending Mr Trump's decision to send the US national guard to the southern border.
He also vowed that "all options" are on the table regarding a US response to a suspected chemical attack in Syria, and defended Mr Trump's comments that he hopes to soon withdraw US troops from that country.
"For too long and the United States of America has been taken advantage of in their responsibility to provide security for the entire world.. American troops aren't going to fix the six or seven different ongoing conflicts and wars going on in the Middle East or in Syria at this stage".
US Department of Homeland Security Kirstjen Nielsen said in a statement: "Since the beginning of this Administration, Tom Bossert has provided wise counsel to the President on Homeland security issues. Over the last fifteen months I have enjoyed working with Tom on a range of current and emerging threats to our nation - particularly in cyber security and disaster response. We at DHS thank him for his partnership and service and wish him the best as he takes new steps in his career."