Bossert was hired as assistant to the president for homeland security and counter-terrorism in December 2016. He has also served as a top White House adviser on cybersecurity issues and natural disasters.
"The president is grateful for Tom's commitment to the safety and security of our great country," Sanders said in a statement. "Tom led the White House's efforts to protect the homeland from terrorist threats, strengthen our cyber defenses, and respond to an unprecedented series of natural disasters. President Trump thanks him for his patriotic service and wishes him well."
Sanders did not give a reason for Bossert's departure, which came one day after John Bolton officially became national security adviser.
Bossert recently was criticized for defending the president's controversial comments from white supremacist protesters in Charlottesville, Va., to Russian interference in the 2016 election. Monday, Bossert said "no voter in this country was influenced" by Russia during the 2016 vote.