"Mitt Romney has shown throughout his life in the private sector, as leader of the Olympics, as governor and in this campaign that he will not back down from difficult challenges," Thune said in a statement. "His plans to revitalize the private sector and restore our country's fiscal health are drawn from his 25-year career as a conservative businessman."
Thune has been a rising star in the Republican Party since winning his seat in 2004 by defeating Democratic Leader Tom Daschle. He had been weighing a White House bid of his own earlier this year before opting to stay in the Senate, the 2012 Iowa Caucuses blog reported.
"On the issues that I have been fighting for in my campaign -- creating a better business environment, lessening the regulatory burden and ending Washington’s spending addiction -- Senator Thune has been a leading voice in the Senate," Romney said.
Thune will appear with the former Massachusetts governor in Des Moines to make the official announcement.
Romney has scored several key endorsements this week, including Sen. Kelly Ayotte (R-N.H.) and Rep. Charlie Bass (R-N.H.). The presidential candidate is reportedly considering choosing Ayotte to be his running mate. Bass was named a national adviser to the campaign.
HuffPost Politics brings you the top political stories three days a week. Learn more