The evacuation was ordered about 4 p.m. local time and the Transportation Security Administration issued an all-clear email shortly after 6 p.m., the news website AL.com reported.
The website said reports indicated the threat was in the form of a written note. Birmingham police Sgt. Johnny Williams Jr. said authorities made the decision to empty the airport and bomb-detecting dogs were used to sweep the airport.
Williams said the airport was reopened after authorities found no evidence of explosives, AL.com said.
The website said a number of flights were delayed or diverted because of the threat.