An "initial investigation" shows that Sarina Esmaeilzadeh had "committed suicide", says Mizan Online, the judiciary's website.
Iranian authorities have denied reports security forces killed a 16-year-old girl during protests ignited by the death of a woman in police custody, Iranian media reported.
The initial probe found that Sarina Esmaeilzadeh killed herself by falling off a roof, media reports said on Friday.
A wave of unrest has rocked Iran since September 16 over the death of Mahsa Amini, 22, after morality police arrested her in Tehran for allegedly failing to observe the country's strict dress code for women.
Mizan Online, the judiciary's website, said late Thursday that "hostile media" reported Sarina Esmailzadeh "had been killed by security forces at a rally" in Karaj, the regional capital of Alborz province.
Rights group Amnesty International said on September 30 that Esmailzadeh, 16, "died after being severely beaten in the head with batons" a week earlier.
Alborz province prosecutor Hossein Fazeli Harikandi said that an "initial investigation" shows she had "committed suicide", Mizan reported.
Esmailzadeh "jumped 20 minutes after midnight on September 24" from "a building situated near her grandmother's house" in the northeast of the city, Harikandi added.
"According to the medicolegal report, the cause of death was a shock caused by the impact," the prosecutor was quoted as saying.
"There had not been any riots in the part of Karaj where this incident happened."
On September 25, Tasnim news agency reported the arrest of "riot leaders" in "several districts of Karaj including Rajaishahr" adjacent to Azimieh where Harikandi said the girl died.
Mizan Online released a short video of Esmailzadeh's mother on Friday, in which she says that her daughter "had nothing to do with" the protests.
The judiciary on Wednesday also said the death of another 16-year-old girl, Nika Shahkarami, was not linked to Amini protests. She went missing on September 20 after a protest in the capital.
In a video sent on Thursday to foreign-based Persian-language media, Shahkarami's mother blamed the authorities for her death.
The demonstrations across Iran, dubbed "riots" by the authorities, have led to dozens of deaths -- mostly of protesters but also of members of the security forces -- and hundreds of arrests.