The siren is one of 96 located in the 10-mile radius around the nuclear power plant. It sounded for about 30 seconds shortly after 1 p.m.Thursday.
Dauphin County Emergency Management Agency officials were notified through an automated monitoring network that the alarm went off. They're working with Exelon, the operator of Three Mile Island, to determine the cause of the false alarm.
Exelon plans to replace all 96 sirens later this year with newer, louder sirens, each with battery backup power. They say the control and monitoring systems for the sirens will also be replaced.