The man was arrested suspected of stabbing three women in the Paris metro on Friday as the capital's end-of-year festivities were in full swing.
The three victims were attacked at three different locations along the Line 3 metro track that runs across central Paris. They were injured, but not critically.
Continued police custody was not appropriate for the suspect because it is "considered incompatible with the state of his health", the prosecutors' office told AFP.
Police had used surveillance-camera footage and mobile-tracking tools to locate the 25-year-old suspect in the Val d'Oise region north of Paris.
Transport police had opened an investigation into attempted homicide and assault with a weapon.
The interior ministry initially said that the man was a Malian citizen who had been ordered to leave France - however four days later it admitted that he is in fact a French citzen.
Interior Minister Laurent Nunez last week called for "maximum vigilance" during the festive season in a message to senior officials due to what he said was a "very high level of the terrorist threat" and "the risk of public disorder".
Nunez specifically asked for particular attention to be paid to security on public transport.
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