Updated at 2:52 p.m.:
Wagner will run in the Second Congressional District, making her announcement following Akin's official word earlier today.
"The Second Congressional District is the place where my husband Ray and I grew up, went to school, raised our children, work and go to church," Wagner said in a statement on her website. "It is the community that has given me extraordinary opportunities to make a difference. And it is the place where I believe I can effectively represent with conservative leadership and values."
Wagner is the second Republican to officially in the race following an announcement last week by St. Louis attorney Ed Martin.
Both Wagner and Martin were present Tuesday for Akin's Senate announcement. Akin declined to endorse a successor, saying: "There's some really, really competent people who are taking a look at that race."
Republican state Sen. Jane Cunningham also has said she is likely to run for Akin's seat.
Updated at 1:45 p.m.:
Former St. Charles County state representative Joe Smith announced on his Twitter page that he is "seriously looking at Congressman Todd Akin's open seat." First-term state Senator Jane Cunningham of Chesterfield said last week that she would likely enter the Congressional race if Akin ran for Senate.
Neither appeared to have created a federal campaign committee.
Will be updated.
Missouri Congressman Todd Akin is entering the Republican primary to challenge Democratic U.S. Sen. Claire McCaskill next year.
Akin described himself as a "consistent conservative" while announcing his candidacy Tuesday at a news conference in the St. Louis suburb of Creve Coeur.
Akin has represented a U.S. House district in suburban St. Louis for the past 10 years and previously served for a decade in the Missouri House. He said today that he has the exact opposite record as McCaskill.
"And we give people a clear choice," Akin said. "It is a choice of two futures for America, and I believe that the people in Missouri will choose the choice of limited government, living within our means, developing American energy, and of course, freedom."
He joins former state Treasurer Sarah Steelman as the only Republicans officially in next year's Senate race.
The Hill's Ballot Box campaign blog shares Steelman's reaction to Akin's announcement:
Steelman was quick to issue a statement decrying the "status quo in Washington."
"The Washington establishment has been looking for a candidate that will protect their interests instead of those of the people," she said in a statement. "I welcome Congressman Akin to the race and look forward to a vigorous debate so we can let the voters decide who is best to stand up for working Missouri taxpayers vs the Washington special interests."
Akin starts with a financial advantage, having outraised Steelman by more than two-to-one last quarter. He had more than $900,000 banked as of the start of April, compared with less than $200,000 for Steelman.
St. Louis businessman John Brunner also is considering a Senate campaign.