Homeland Security Secretary Kristi Noem is stepping down at the end of the month, President Donald Trump said Thursday, saying that he would tap Sen. Markwayne Mullin, R-Okla., to replace her.

“I am pleased to announce that the Highly Respected United States Senator from the Great State of Oklahoma, Markwayne Mullin, will become the United States Secretary of Homeland Security (DHS), effective March 31, 2026,” Trump wrote in a post on Truth Social.

He added that Noem, who he said “has served us well,” will take over a new role called “Envoy for The Shield of the Americas.”

The president described that position as one that will lead “our new Security Initiative in the Western Hemisphere.”

Trump spoke to Noem this afternoon about the job switch, according to two people familiar with the conversation. The secretary is traveling outside of Washington, D.C.

Noem has been at the forefront of Trump’s signature policy agendas: the deportation of immigrants, the restriction of immigration and the clamping down of the U.S.-Mexico border. Her ouster marks the first time a Cabinet secretary will step down in Trump’s second term. Last year, Mike Waltz left his Cabinet-level post as national security adviser after Trump nominated him to become ambassador to the United Nations.

Noem’s place in the administration became increasingly unstable following the fatal shootings of two U.S. citizens by federal agents during immigration operations in Minneapolis, her fraying relationship with the U.S. Coast Guard, the only branch of the military under her command, and her comments this week before lawmakers that rankled Trump.

In the congressional hearings on Tuesday and Wednesday, lawmakers questioned Noem on the $200 million ad campaign she oversaw that urged anyone in the U.S. illegally to deport voluntarily. The ad campaign, which was conducted mostly in English, featured Noem. According to AdImpact, the Department of Homeland Security has spent almost $80 million to air these ads since the start of 2025, not including the cost of production.

Noem on Tuesday told the Senate Judiciary Committee that the president approved the multimillion-dollar ad campaign, a claim that the White House denied.

“POTUS did not sign off on a $220 MILLION dollar ad campaign. Absolutely not,” a White House official told NBC News on Thursday.

At Senate and House hearings this week, Noem also faced questions from lawmakers over the administration’s actions in Minneapolis. The deployment of thousands of federal troops to conduct immigration enforcement activities, using at times brutal tactics, has led to intense bipartisan criticism of Noem’s leadership of DHS.

Noem declined to apologize for calling Renee Good and Alex Pretti, two Americans who have been killed by federal agents, “domestic terrorists,” though she conceded that investigations into the two deaths are ongoing.

Republican Sens. Thom Tillis and Lisa Murkowski in January called for Noem to step down, with Tillis telling reporters he couldn’t “think of any point of pride over the last year” of her tenure.

Before serving in the Trump administration, Noem was the governor of South Dakota.

Earlier Thursday, Mullin declined to answer questions from NBC News about whether he had spoken to the president about taking over the homeland security secretary role.

The senator simply said that he’d spoken to Trump “recently” and added, “I just don’t want to talk about that right now. We’ll discuss it later.”

Mullin, a former mixed martial arts fighter, served for a decade in the House before he won a 2023 special election to the Senate.