Three Mile Island Nuclear Generating Station (TMI) is a closed nuclear power plant on Three Mile Island in Londonderry Township, Pennsylvania, on the Susquehanna River just south of Harrisburg. It had two separate units, TMI-1 (owned by Exelon Generation) and TMI-2 (owned by FirstEnergy Corp). The plant was the site of the most significant accident in United States commercial nuclear energy when, on March 28, 1979, TMI-2 suffered a partial meltdown. According to the Nuclear Regulatory Commission (NRC) report, the accident resulted in no deaths or injuries to plant workers or in nearby communities. Follow-up epidemiology studies have linked no incidents of cancer to the accident.

The reactor core of TMI-2 has since been removed from the site, but the site has not been decommissioned. In July 1998, Amergen Energy (now Exelon Generation) agreed to purchase TMI-1 from General Public Utilities for $100 million.

It is named Three Mile Island because it is located three miles downriver from Middletown, Pennsylvania. The plant was originally built by General Public Utilities Corporation, later renamed GPU Incorporated. The plant was operated by Metropolitan Edison Company (Met-Ed), a subsidiary of the GPU Energy division. During 2001, GPU Inc. merged with FirstEnergy Corporation.

Exelon was operating Unit 1 at Three Mile Island at a loss since 2015. In 2017 the company said it would consider ceasing operations at Unit 1 because of high costs unless there was government action; it shut down in 2019.

Unit 2, which has been dormant since the accident in 1979, is still owned by FirstEnergy, and it is estimated to close in 2037.