The Olympian-turned-reality star, who is perhaps the most high-profile face of the transgender community, at first was quiet after the Trump administration withdrew Obama-era guidance on transgender bathroom use in public schools.
But she broke her silence over 24 hours later in a tweet and video message.
Clad in a pink "pussy bow" blouse, Jenner spoke directly to the camera.
"I have a message for President Trump from well, one Republican to another. This is a disaster. And you can still fix it. You made a promise to protect the LGBTQ community," she said, holding up her hand like a mock telephone. "Call me."
After Trump became the presumptive GOP nominee in June, Jenner told STAT she believed he would be better for women than Hillary Clinton.
"Everybody looks at the Democrats as being better with these [LGBT] issues. But Trump seems to be very much for women. He seems very much behind the LGBT community because of what happened in North Carolina with the bathroom issue. He backed the LGBT community. But in Trump's case, there's a lot more unknowns. With Hillary, you pretty much know what you're gonna get with the LGBT community," Jenner said at the time.
She was spotted attending an inauguration eve dinner at Washington's Union Station alongside House Speaker Paul Ryan.
"Republicans need help understanding LGBTQ issues and I'm here to help!" she tweeted.
Jenner, who came out as transgender on the cover of Vanity Fair magazine in 2015, documented her life after transition in two seasons of the E! docuseries "I am Cait." The show often got political; Jenner frequently discussed transgender rights and featured LGBT activists in nearly every episode.
Her video Thursday also included a message for transgender kids -- and bullies, even taking a shot at Attorney General Jeff Sessions.
"I have a message for the trans kids of America. You're winning. I know it doesn't feel like it today or every day, but you're winning. Very soon we will win full freedom nationwide and it's going to be with bipartisan support. You can help by checking out the National Center for Transgender Equality and letting Washington hear you loud and clear," she said. "Now I have a message for the bullies: You're sick. And because you're weak, you pick on kids, you pick on women or anyone else you think is vulnerable. Apparently even becoming the attorney general isn't enough to cure some people of their insecurities."
Jenner is not the only celebrity to speak against the new policy: singer Jackie Evancho, who performed the national anthem at Trump's inauguration and has a transgender sister, tweeted
the President on Wednesday evening asking for a meeting. Talk show host Ellen DeGeneres also weighed in on Twitter, writing
, "For anyone who feels scared or hopeless, know that we will never stop fighting for your rights."