In a statement emailed by the Romney campaign, Thune, a potential vice presidential short-lister, said Romney's economic prowess led him to endorse the former Massachusetts governor.
"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," he said. "His plans to revitalize the private sector and restore our country's fiscal health are drawn from his 25-year career as a businessman."
Thune's endorsement comes after another high-profile senator, Kelly Ayotte of New Hampshire, also announced her support for Romney.
But an endorsement that will not be coming out shortly for Romney is one from his 2008 adversary -- current Fox News host Mike Huckabee.
After C-Span's political director Steve Scully tweeted that "sources" indicated the former Arkansas governor was set to endorse Romney, Huckabee said the report was flat out wrong.
"The absolute fabrication of it makes that 'source' as reliable as the Three Stooges as a source for how to understand physics," Huckabee told Fox News.
"Where people get this stuff is beyond me. I'm not endorsing anyone and if I do, the release will be directly from me --not some anonymous goon," Huckabee added.
Huckabee is hosting eight presidential GOP candidates at an Iowa forum next month that will center around pro-life and other socially conservative issues. Romney is scheduled to attend.