The move marks a shift in his public stance, after his not taking a hard line against transgender rights during the campaign.
Trump never specifically talked about the policy during the campaign, instead tending to talk about LGBT rights in relation to news events that were playing out at the time.
The 'bathroom bill'
He first spoke specifically about transgender rights when the controversial "bathroom bill" went into effect in North Carolina during the campaign.
Though not committing either way, Trump called it a "very strong" move to force people to use the sex indicated on their birth certificate to determine which bathroom they used, while noting that the state was "paying a big price" for implementing it.
Those comments came during an April 21, 2016, appearance on NBC's "Today," during which he also said Caitlyn Jenner could use any bathroom she wanted if she visited Trump Tower in New York.
Despite Trump's earlier comments, his administration rescinded a guidance issued to schools by the Obama administration to allow students to use bathrooms that match their gender identity rather than the sex indicated on their birth certificate.
The reversal was announced in a letter to schools from the Justice and Education departments on Feb. 22, 2017, saying the Obama-era directive led to confusion and lawsuits over enforcement.
Reaction to Orlando shooting
"Thank you to the LGBT community! I will fight for you while Hillary brings in more people that will threaten your freedoms and beliefs," he wrote in a June 14, 2016, tweet.
"Only weeks ago, in Orlando, Florida, 49 wonderful Americans were savagely murdered by an Islamic terrorist. This time the terrorist targeted our LGBT community. As your president, I will do everything in my power to protect our LGBT citizens from the violence and oppression of a hateful foreign ideology," Trump said.
Before his official run
In 2000, when Trump publicly toyed with — and later dismissed — the idea of a presidential run that year, he was interviewed by The Advocate, an LGBT magazine.
While he was not specifically asked about transgender Americans' serving in the military, he said he would be supportive of gay people serving in the military.
"If a gay person can be a doctor or a lawyer or a teacher or take another position of responsibility, why can't they serve this country in the military?" Trump said.
"'Don't ask, don't tell' has clearly failed. Gay people serve effectively in the military in a number of European countries. There is no reason why they can't serve in the United States," he added.
When it came to his administration, he said he would want "the best and the brightest."
"Sexual orientation would be meaningless. I'm looking for brains and experience. If the best person for the job happens to be gay, I would certainly appoint them," he said in the interview.