President Donald Trump said on Twitter the U.S. will not "accept or allow" transgender people "to serve in any capacity in the U.S. Military."
Trump said in a series of tweets Wednesday he consulted with generals and military experts on the issue.
Trump's announcement comes a little more than a year after then-Secretary of Defense Ash Carter lifted the ban on transgender people openly serving in the military.
The Pentagon was supposed to start letting transgender recruits enlist this year.
But last month, current Defense Secretary Jim Mattis said he was delaying the new policy because he needed more time.
Transgender people already serve in the military, and it's not yet clear how Trump plans to enforce the new ban.