Rounds, 58, said he plans to challenge three-term Democratic Sen. Tim Johnson, The Argus Leader of Sioux Falls, S.D., reported Thursday.
"I am here today to ask the people of South Dakota for their support and to allow me to work for them as their United States Senator in 2014," Rounds said in a statement.
Johnson, 65, hasn't officially said if he'll seek a fourth six-year term, but hinted he will.
"As in past campaigns, I will make my formal announcement later next year," Johnson said in a statement. "But I feel great, still have work to do, and I fully intend to put together a winning campaign in the weeks and months ahead."