"We know that it is a clearly anti-Semitic act, a serious act committed a few dozen metres (yards)" from a Jewish school, police chief Laurent Nunez told reporters.
The attack against the 35-year-old teacher occurred in broad daylight in front of witnesses in the south of the port city, police said.
The teacher was on his way to work at the Franco-Hebraic Institute when the assailant, around 15 years old, attacked him.
"The individual did not seem to possess all of his faculties," a source close to the case told AFP.
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Another source said: "This is a very excited individual, but one who clearly claims responsibility for his act, which you can describe as anti-Semitic."
Interior Minister Bernard Cazeneuve reacted with a tweet saying: "Support for the victim of the revolting anti-Semitic aggression in Marseille".
The attack came nearly two months after another assault, north of Marseille, in which three people shouting anti-Semitic obscenities and expressing support for the Islamic State group stabbed a Jewish teacher, injuring him in the arms, legs and stomach.