The group, HaMoked, said the 14-year-old girl, named only as Ghada, was arrested in Jerusalem on 13 January without a permit while visiting the Israeli-controlled city.
HaMoked said the girl was told she would be dropped off at a checkpoint near her home but on her release on 16 January instead was taken to the Erez crossing into Gaza.
Abir Joubran-Dakwar, HaMoked’s lawyer dealing with the case, said they had not yet received a response from Israel about when she could return.
She said Ghada had never been in Gaza before and did not know anyone there, though she was now staying with members of her extended family.
“No one told her anything – they told her they would send her to Qalandia [a checkpoint near her home] but they sent her to Erez.”
She said Ghada had epilepsy and was concerned the stress could affect her condition.
Joubran-Dakwar said it was the first time she had heard of someone as young as 14 being sent there.
It is extremely difficult for Palestinians from the West Bank to obtain Israeli permission to visit the strip.