Emergency services were called to the prison at 3.30pm on Tuesday, where they found three male prisoners suffering from stab wounds.
One of the prisoners, a man aged in his 20s, was pronounced dead at the scene at 4.25pm.
Police are in the process of informing his family and he has yet to be formally identified.
The two other men, aged 21 and 30, are in a critical condition at an east London hospital, police said.
Two prisoners, aged 26 and 34, have been arrested and taken to a police station in north London for questioning.
A Prison Service spokesman said: "Police are investigating an incident at HMP Pentonville this afternoon.
"It would be inappropriate to comment further at this stage."
A Met Police spokesman said: "Detectives from the Homicide and Major Crime Command are investigating.
"Further enquiries into the incident continue."
The category B prison was opened in 1842 and holds more than 1,200 adults.
