One man died and eight others were injured after a fight between two groups of inmates broke out in the psychiatric unit of maximum-security prison Korydallos in Athens late on Thursday.
The inmates had make-shift knives, media report.
The fight broke out between Albanian, Russian-speaking Greeks and Georgian inmates.
The injured inmates were transferred to hospitals in the city. One of them is in serious condition.
A prison guard was also slightly injured, the ministry of Justice said in a statement.
The fight lasted no more than 5 minutes, with the majority of psychiatric unit inmates to participate in it.
Greece has some of the most overcrowded prisons in the European Union. But the number of inmates per cell has improved in recent years due to prisoners having more opportunities for early release, as well as the construction of new prisons and additions to existing ones.