He said an active shooter drill planned for Wednesday would not take place because officials learned during the lockdown that the system worked as designed.
Authorities asked people to avoid the Sparkman Center.
Redstone Arsenal boasts some 40,000 workers, primarily civilian and contract employees.
Redstone Arsenal hosts units of the army missile command as well as a NASA research centre.
Chris Colster, a spokesman for the Army post, said he was not aware of any casualties but could not definitely confirm there weren't any.
The Arsenal's Twitter account posted that the "installation is locked down".
There are no reports of injuries at this time.
The gates were reopened at 12:30 p.m. with the note that everyone should avoid the area around the Redstone Arsenal Sparkman Complex.
The governor's office says Alabama Gov. Kay Ivey is being updated about the situation, but her office couldn't provide any details. He also said Redstone Arsenal was scheduled to hold active shooter training this week.
The base was placed on lockdown amid reports of a possible active shooter. In late 1956 the Army was relieved of most of its ballistic missiles in favor of similar weapons operated by the US Air Force.
Redstone Arsenal is located in Madison County, Ala., which is south of Huntsville near the Huntsville International Airport.