An FBI agent was injured when a flash grenade inadvertently went off inside a Manhattan federal building Wednesday afternoon, according to officials.

The agent accidentally discharged the grenade in his department car at the garage of 26 Federal Plaza around 1:45 p.m., a police source said.

“A small incendiary device went off inadvertently,” another source told The Post.

The man was taken to Bellevue Hospital, where he was in stable condition with an injury to his right hand and wrist, a law enforcement source said.

The building was not being evacuated, officials said, and the explosion didn’t appear to be connected to terrorism, a law enforcement official said.