Neighbors said the mass killing, inside an apartment in the capital of St. Helier, involved members of the same family.
Five were declared dead at the scene while a sixth person died while undergoing emergency surgery at Jersey Hospital, police said. They said the victims were two men, two women and two young children. They said the murder suspect also sustained unspecified injuries that were not life-threatening.
Police said the victims were Jersey residents. Some neighbors said they thought at least some of the victims were Polish.
One witness, Andre Thorpe, said he saw police running around the property trying to gain access, then paramedics carrying a bloodied child from the scene.
Jersey's head of crime services Stewart Gull, who is leading the investigation, said detectives haven't established the circumstances of the attack.
"Clearly this complex investigation is in its very early stages as we try to establish exactly what happened," Gull said.
Violent crime is rare on Jersey, a British island dependency about 15 miles (20 kilometers) off the west coast of France's Normandy peninsula.