Armitage's official announcement, expected later in the day, came a day after his boss, Secretary of State Colin Powell announced he was leaving the State Department.
Armitage's departure was widely expected. On Monday, department spokesman Richard Boucher said that the two men were part of a "very successful team."
"It's like Bosnia. In together, out together," Boucher added.
Powell said Monday he would stay in the job until a successor, national security adviser Condoleezza Rice, is in place. Armitage is expected to stay in the job until then, the official, who spoke on condition of anonymity, said.
President Bush is expected to nominated Rice for the top State job Tuesday.