The abrupt ban on transgender people serving in the US military was announced by the US President on Twitter.
At least one openly transgender member of the military has vowed to keep serving despite the ban.
“I would like to see them try to kick me out of my military,” Staff Sergeant Logan Ireland, of the Air Force, told the Air Force Times.
“You are not going to deny me my right to serve my country when I am fully qualified and able and willing to give my life.”
“We have pilots, we have doctors, we have combat medics, we have security forces members like myself,” Mr Ireland, who is married to Army Corporal. Laila Villanueva, a transgender woman, said.
“We are everywhere in the military, and for our president to not have a military member’s back that is willing to die for him blows my mind. And it just makes me very motivated today to continue my training.”
Retired Army Sergeant Shane Ortega, who was the first out transgender soldier to serve in the US military, denounced Mr Trump as a "war dodger" with "no connection to reality."
He told Mic: “To be quite frank, my first thought was, ‘F**k!’”
"Trump is a man who is literally a war-dodger, who comes from a life of privilege and silver spoon, who has no connection to reality whatsoever, and I don’t know how much more the American public needs to see that.”
Mr Trump received five deferments from the military draft during the Vietnam War, four for educational purposes, and one for having a bad foot.
It is unclear what the ban will mean for currently-serving transgender soldiers.
A Rand Corporation study estimated there were between 1,320 to 6,630 transgender people currently serving in the US military.
Major advocacy groups depicted Mr Trump's pronouncement as an appeal to the portion of his conservative base that opposes the recent civil-rights gains by the LGBT community.
"His administration will stop at nothing to implement its anti-LGBTQ ideology within our government - even if it means denying some of our bravest Americans the right to serve and protect our nation," said Sarah Kate Ellis, president of the LGBT-rights group Glaad.
Transgender military members have been able to serve openly since last year, after a move by then-defence secretary Ash Carter.
Mr Trump's vow to end that policy was the latest, and perhaps the most stinging, of a string of actions since his election which have dismayed supporters of LGBT rights.
Additional reporting by AP
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