This directive overturns an order signed by Joe Biden in January, which aimed to extend Harris's security until January 2026.
Normally, Kamala Harris's Secret Service protection would have concluded on 20 July, six months after her term ended.
Trump has a history of using his authority over the Secret Service to withdraw protective details from individuals he perceives as political adversaries.
Previous instances include the removal of protection for John Bolton, Mike Pompeo, and Joe Biden's adult children, Hunter and Ashley Biden.