Rounds bested four other Republicans for the nomination: physician Annette Bosworth, state Rep. Stace Nelson, war veteran Jason Ravnsborg and state Senate Majority Whip Larry Rhoden.
Businessman Rick Weiland was unopposed in the Democratic primary, though he visited every town in the state as he campaigned.
Rounds and Weiland will be in a four-way race in November with former Republican Sen. Larry Pressler and former GOP state Sen. Gordon Howie, who are running as independents.
The Huffington Post's Amanda Terkel and Samantha Lachman took a look at the race earlier this week:
Former Gov. Mike Rounds (R) has been the GOP frontrunner in the race to replace retiring Sen. Tim Johnson (D-S.D.), although state Rep. Annette Bosworth (R) has been getting a fair amount of attention in the five-person Republican primary. In April, she likened food stamp recipients to wild animals. Still, Bosworth has lagged in the polls and her campaign has been in debt. Rounds, meanwhile, has focused on the failings of Washington and said Congress needs to focus more on debt reduction. A candidate must get at least 35 percent of the vote in order to receive the nomination, otherwise a run-off between the top two finishers will be scheduled. The winner will face Democratic businessman Rick Weiland, who was once a staffer for former Sen. Tom Daschle (D-S.D.).