It was not immediately clear why Mr Bossert stepped down. White House Press Secretary Sarah Huckabee Sanders said Mr Trump was “grateful for Tom’s commitment to the safety and security of our great country”.
“President Trump thanks him for his patriotic service and wishes him well,” she said in a statement.
The adviser’s resignation came just one day after incoming national security adviser John Bolton took office, bringing with him the possibility of a staff shakeup on the National Security Council (NSC).
Mr Bossert, an ally of former national security adviser HR McMaster, was on uncertain footing with his new boss, according to the Washington Post. Several outlets reported that Mr Bossert was forced out by the incoming national security adviser, in a move that surprised both him and several of his colleagues on the NSC.
A White House official said Rob Joyce, the current NSC cybersecurity coordinator, would take Mr Bossert’s place on an acting basis, according to the New York Times
Mr Bossert joined the administration under former national security adviser Michael Flynn, who was later fired and charged with lying to the FBI. Mr Bossert was well-liked by the president, according to the Post, and regularly appeared on television to defend Mr Trump’s agenda.
The adviser worked on a broad array of security issues during his tenure, including terrorism, natural disasters, and cyber attacks. He recently addressed possible responses to a suspected chemical attack in Syria, saying all options remained on the table.
Mr Bossert was perhaps most visible during a spate of hurricanes this summer, when he was frequently called upon to defend the Trump administration’s response to the disasters.
Addressing criticism of the federal response to Hurricane Maria in Puerto Rico, Mr Bossert claimed that news coverage “in some cases, is giving the appearance that we’re not moving fast enough”.
“But what I will tell you is that we are mobilising and marshalling the resources of the United States of America in a way that is absolutely professional, fast and adequate to meet the needs,” he said.
Six months after the hurricane struck, 16 per cent of the island remained without power.