Rep. Mike Waltz, R-Fla., retired from Congress Monday to serve as President Trump's national security adviser – a long-awaited move that he announced just one hour before Trump was sworn in as president.

Waltz had confirmed to Fox News his plans to resign on Inauguration Day prior to Trump's inaugural ceremony, a decision that further narrows the Republican House majority to just 218–215.

"As I prepare to assume my duties as National Security Advisor to the president-elect, Donald J. Trump, I hereby make my resignation official from the U.S. House of Representatives, effective January 20th, 2025," Waltz said in a letter announcing his decision.

"I intend to serve out my term in the 118th Congress, and to be sworn into the 118th Congress. I have also submitted by notice to Governor Ron DeSantis to advise him on my resignation."

A special election to replace Waltz will be held in April. 

News of his resignation in the House comes at a time when Republicans in the chamber have struggled to unify amid intraparty divisions over government spending, fealty to Trump, and other differences in ideology. 

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These divisions were on sharp display during the election of House Speaker Mike Johnson, R-La., earlier in January and the massive spending bill in December that narrowly averted a government shutdown. 

Waltz, a former Green Beret who served in both the White House and Pentagon, was tapped by Trump in the fall of 2024 to serve as his national security adviser. 

Trump praised Waltz's more than 25 years in the Army Special Forces, noting that he "was deployed multiple times in combat for which he was awarded four Bronze Stars, including two with Valor."

Fox News Digital's Haley Chi-Sing contributed to this report.