This directive revokes an order signed by former president Joe Biden in January, which had extended Harris's security until January 2026.
Under normal circumstances, Harris's protective detail would have ended on 20 July, six months after her term concluded.
The order comes ahead of Harris’s planned tour to promote her new memoir, 107 Days, in September.
Trump has previously used his authority over the Secret Service to remove protective details for perceived political adversaries, including John Bolton and Mike Pompeo.