Trump has directed the military to reverse an Obama-era policy that allowed transgender people to serve openly in the military. It would have gone into effect this year.
It also said service members couldn't be discharged or denied re-enlistment just because of their gender identity.
In late July, Trump released a series of tweets saying he would not allow transgender individuals to serve in the U.S. military "in any capacity." A month later, the White House put out a memo with guidelines on what that ban would look like.
On Monday, the American Civil Liberties Union sued the government on behalf of several transgender individuals serving in the military. They say the ban is a discriminatory violation of their constitutional rights to equal treatment and due process.
Basically, the lawsuit argues kicking transgender people out of the military breaks the Constitution's mandate that they be treated the same as everyone else under the law.
Trump did try to legally justify his ban by arguing transgender service members place a "tremendous" burden in medical costs on the military.
But a RAND study suggests that's not quite true. It found the Department of Defense only spent about one-hundredth of a percent of its total budget on gender-related care for transgender service members.
Forbes later reported the military spends about 10 times that amount on erectile dysfunction medications like Viagra.
This isn't the only lawsuit Trump is facing due to the ban. Lambda Legal also filed a suit Monday on behalf of two transgender people who want to join the military and one current member who hopes to become an officer.
And five other transgender service members sued the administration in early August, before the specifics of the ban were laid out. The White House has said it won't comment on pending litigation.