Flynn's departure less than one month into the Trump administration marks an extraordinarily early shakeup in the president's senior team of advisers. Flynn was a loyal Trump supporter throughout the campaign, but his ties to Russia caused concern among other senior aides.
Trump named retired Lt. Gen. Keith Kellogg as the acting national security adviser. Kellogg had previously been appointed the National Security Council chief of staff and advised Trump on national security issues during the campaign.
Flynn, 57, served as an advisor to Trump during his presidential campaign and was highly critical of Democratic presidential candidate Hillary Clinton and her use of a private email server while secretary of state.
He is a retired U.S. Army three-star general, was assistant director of national intelligence under President Barack Obama, but was fired in 2014. In Flynn's book, "The Field of Fight," Flynn claimed he was fired for speaking out against policies of the Obama administration, but others claim Flynn was pushed out because of his combative management style.