A flash grenade was a type of non-lethal weapon used by Starfleet in the early 24th century. A flash grenade produced a brief, intense flash of light capable of temporarily disorienting most humanoid species.

Starfleet flash grenades were first used against Cardassian targets running a raid on an illegal Cardassian base on the moon of Delavi III in 2328. In that encounter, the Cardassians proved quick to recover from the weapon's effects. (CoE eBook: Turn the Page)

These were occasionally also called photonic grenades because of their output of light, though there was no technical connection to the photonic torpedo. (SCE eBook: Fatal Error)