A spokesman for the state Department of Corrections and Rehabilitation says two inmates repeatedly stabbed the victim Saturday in a yard at Salinas Valley State Prison in Soledad.
Jonathan Parsley said an unknown number of other inmates in the yard then began to attack each other.
Four inmates suffered injuries from the brawl, two of whom had to be airlifted to a hospital. The extent of their injuries was not immediately known.
The name of the dead inmate is being withheld pending notification of his relatives.
Authorities have limited inmate movement at the maximum-security prison as an investigation into what happened continues.