The station says the gun was left on top of a toilet paper dispenser by David Evans, a staffer for Missouri House Speaker Tim Jones (R-Eureka).
The House Minority Leader, Jacob Hummel (D-St. Louis), says the incident shows a disregard for the safety of those who visit the capitol, including school children.
"Thank goodness no one was hurt," he told CBS St. Louis. "The public is not allowed to carry concealed weapons into Missouri government buildings. It's another case of legislators thinking they are above the law."
Hummel said it might be time to revisit the state's concealed carry laws.
The state legislature passed a law in 2011 allowing lawmakers and their staff to carry concealed guns in the capitol.
But Minority Leader Hummel says anyone who has taken the required concealed carry course should know better. The staffer, Evans, does have a conceal carry permit and the gun was given back to him.
CBS St. Louis reported that calls to both Evans and House Speaker Jones were not returned.