The France international was arrested Wednesday morning following an assault that took place last week.
Sources have told ESPN that the police investigation relates to an attack on PSG teammate Kheira Hamraoui on Thursday.
Sources added that on that night, Hamraoui was in Diallo's car with two other PSG teammates after leaving a club dinner in Paris.
As the car pulled out in front of her home, two men wearing balaclavas armed with iron bars dragged Hamraoui out of the car and started hitting her on the legs, according to sources. The assault lasted two minutes before the men escaped.
PSG confirmed that Diallo was placed in police custody but did not publicly name the victim.
A club statement read: "Paris Saint-Germain acknowledges that Aminata Diallo was remanded in police custody this morning by the Versailles SRPJ as part of the proceedings initiated following an assault last Thursday evening against the club's players.
"Paris Saint-Germain condemns in the strongest possible terms the violence committed. Since the evening of Thursday, Nov. 4, the club has taken all the necessary measures to guarantee the health, well-being and safety of its players.
"PSG is working closely with the Versailles SRPJ to shed light on the facts. The club is paying close attention to the progress of the proceedings and will study what action to take."
Sources added that Hamraoui was taken to hospital after the attack and required stitches for her leg injuries. She has not sustained career-threatening injuries but was unable to make her 100th appearance for the club against Real Madrid in the Champions League on Tuesday.
Instead, Diallo started the match as PSG enjoyed a 4-0 win to earn their third consecutive victory in the competition this season.
Hamraoui and Diallo are both defensive midfielders and are in direct competition for a place in the PSG team and France national side.
Diallo joined PSG in 2016 and had a loan spell with the National Women's Soccer League club Utah Royals for the 2020 season.