Trump will tap Oklahoma Senator Markwayne Mullin to replace her, he said on his Truth Social platform on Thursday, a move that would require US Senate confirmation.
Noem, a former governor of South Dakota, became one of Trump's most high-profile Cabinet secretaries with social media posts that portrayed immigrants in harsh terms, highlighting cases of alleged criminal offenders and using vitriolic language.
During congressional hearings this week, Democrats and some Republicans criticised Noem for her approach to the immigration crackdown and management of her department, including concern over a $US220 million ($A315 million) ad campaign that heavily featured Noem and was awarded to two longtime Republican operatives without a standard bidding process.
Trump told Reuters on Thursday that he did not sign off on the ad campaign prominently featuring Noem, including a scene of her on horseback at Mount Rushmore in her home state of South Dakota.
In one congressional hearing this week, Noem told Republican US Senator John Kennedy that Trump had approved the ad campaign. Noem faced criticism in January when she quickly labelled two US citizens fatally shot by federal immigration agents in Minneapolis as committing "domestic terrorism."
Videos that emerged after the deaths undercut the assertion by Noem and other Trump officials that the two deceased - Renee Good and Alex Pretti - were violent aggressors.
The public backlash for the deaths led the Trump administration to move to a more targeted approach to immigration enforcement in Minnesota after months of sweeps through US cities that led to violent clashes with residents opposing the crackdown.
Democrats in the US House of Representatives moved to impeach Noem and at least two Republicans in Congress called for her to lose her job after the incidents.
Trump said on Truth Social Noem would be appointed envoy to a planned summit in Miami to reinforce his Western Hemisphere policies.
Within minutes of Trump's post about her being replaced, Noem spoke at a law enforcement event in Tennessee for 40 minutes, but did not mention her departure. She acknowledged the move in a post on X later, saying, "We have made historic accomplishments at the Department of Homeland Security to make America safe again."
Mullin, who spent a decade in the House of Representatives before becoming a senator in 2023, also supports Trump's hardline immigration agenda. To become DHS secretary, Mullin would require Senate confirmation.
Speaking to reporters on Thursday, Mullin said he was not expecting the call from Trump. He described Noem as a friend and said he had not had a chance to call her yet.
"She was asked to a very difficult job," Mullin told reporters.
"I think there's an opportunity to build off successes and there's also opportunities to build off things that maybe didn't go quite as planned."
Democrats in Congress have blocked funding for DHS since mid-February, saying federal immigration enforcement must be reformed. Noem's ouster did not appear to break the stalemate.
The popularity of Trump's immigration approach fell as agents detained US citizens and tear-gassed streets in an attempt to drive up deportations, which last year fell short of the administration's goal of one million per year.
Critics said Noem demonised immigrants and promoted an immigration enforcement strategy that targeted non-criminal, working immigrants and families.
with Reuters
