WASHINGTON — Alan Armstrong, a Tulsa businessman, was sworn in Tuesday as Oklahoma’s newest U.S. senator.

Armstrong temporarily fills the seat of Markwayne Mullin, who was sworn in as U.S. Department of Homeland Security secretary earlier Tuesday. 

The Senate on Monday confirmed Mullin’s nomination to lead the agency responsible for carrying out President Donald Trump’s mass deportation agenda. 

Armstrong was sworn in at the U.S. Capitol just hours after Oklahoma GOP Gov. Kevin Stitt appointed him to the post Tuesday morning at the Oklahoma state Capitol in Oklahoma City. 

Iowa GOP Sen. Chuck Grassley, who serves as president pro tempore of the Senate, swore in Armstrong. Grassley joined Armstrong and his family in the Old Senate Chamber for a reenactment of the swearing-in shortly after. 

Armstrong has served as executive chairman of the board of directors for Williams. The major energy company is headquartered in Tulsa. 

Armstrong joins Oklahoma GOP Sen. James Lankford in the Senate and will serve alongside him until January 2027 — the remainder of Mullin’s term. 

Under Oklahoma law, Armstrong signed an affidavit earlier Tuesday vowing to not run for a full Senate term in 2026, the Oklahoma Voice reported. 

Earlier in March, Trump gave Oklahoma GOP U.S. Rep. Kevin Hern — who is running in November for the Senate seat — his “complete and total endorsement.”