The incident took place at around 11.30 pm on Sunday evening in the town of Champs-sur-Marne in the eastern suburbs of Paris.
The stepfather, who was acting under the influence of alcohol, pulled the mother's hair and grabbed her by the neck in their bedroom, according to reports.
“He pushed her on the bed. I told Tonton (“Uncle” in English — the boy's nickname for his stepfather) to stop,” said the 10 year old during his hearing.
“Mom told me to call the police, I cried, I was afraid for her.”
During the argument, the mother said that she had asked her son to get something to release her partner's grip on her throat.
“I went to the kitchen and took a big knife,” the child said.
“I kept screaming at him to stop but he kept going. I stabbed him in the back. Mum bit him on the leg.”
The boy's actions have been considered self-defense by the courts, with his behaviour described as 'courageous' by a spokesperson for the prosecution team.
“He took his courage in both hands,” the spokesperson said.
As a result the stepfather was wounded on the shoulder blade.
On Tuesday, the stepfather — a 36-year-old plumber and heating engineer — was sentenced to a six months suspended sentence and probation for two years under the obligation to cure his addiction to alcohol, as well as to attend anti-domestic violence classes.
According to reports, this was the first time he had attacked his partner and no ban has imposed on contact between the two parties.
The mother did not file a complaint and did not wish to have her injuries reported to the medical-legal unit.
The couple has three other dependent children, including two from a previous union of the mother's, and the questions remains whether the stepfather will return to his partner's home.