Joni Kay Ernst (née Culver; born July 1, 1970) is an American politician and retired military officer serving since 2015 as the junior United States senator from Iowa. A member of the Republican Party, she previously served in the Iowa State Senate from 2011 to 2014 and as auditor of Montgomery County from 2005 to 2011. As Chair of the Senate Republican Policy Committee from 2023 to 2025, after having been vice chair of the Senate Republican Conference since 2019, Ernst was the fourth-ranking Republican in the Senate.

After graduating from Iowa State University, Ernst joined the United States Army Reserve. She served in the Iowa Army National Guard from 1993 to 2015, retiring as a lieutenant colonel. During the Iraq War, she served as the commanding officer of the 1168th Transportation Company in Kuwait and later commanded the 185th Combat Sustainment Support Battalion at Camp Dodge.

After having been Montgomery County Auditor and serving in the Iowa State Senate, Ernst was elected to the U.S. Senate in 2014, becoming the first Republican to win the seat since 1978. She was reelected in 2020.

During her time in the Senate, Ernst has called for reforms to Medicare, Medicaid, and Social Security. During the Trump administration, she expressed concern about Trump's trade war with China, and as a war hawk, criticized some of his foreign policy. While supporting both Trump's nominees for EPA administrator, she expressed concern over their commitment to the Renewable Fuel Standard. She rejects the scientific consensus on climate change.