It is believed to be the first time a virtual sexual offence has been investigated by cops.
The girl, who is under 16, was allegedly left distraught after her online character was attacked by others.
Whilst there was no physical attack, police said the girl suffered psychological and emotional trauma.
The assault took place in the metaverse, the virtual reality world launched by Meta and Facebook boss Mark Zuckerberg.
It is seen as the next major evolution of the internet.
Users wear a virtual reality headset and interact with others via their playable avatars.
The victim was in an online “room” with a large number of users when she was “raped”.
A senior officer working on the case said: “This child experienced psychological trauma similar to that of someone who has been physically raped.
“There is an emotional and psychological impact on the victim that is longer term than any physical injuries.
“It poses a number of challenges for law enforcement given current legislation is not set up for this.”
It is not the first time online avatars have been sexually assaulted.
Several players of Horizon Worlds, a free VR online game run by Meta, have reported being attacked by other players.
Meta said: “The kind of behaviour described has no place on our platform, which is why for all users we have an automatic protection called personal boundary, which keeps people you don’t know a few feet away from you.”