The child, who has not been named, was attacked while walking alone in the northern city of Basel around midday local time on Thursday, police said in a statement.
The boy’s class teacher, who was passing on a bicycle, found him lying seriously injured on the sidewalk and called for an ambulance. The boy died shortly after emergency surgery in hospital, the statement added.
Soon after the stabbing, a 75-year-old woman turned herself in to the police. She admitted attacking the boy and was arrested, police said, but did not explain the motive for the attack.
Police said investigations were continuing and appealed for witnesses.