Last week Mohammadi's family and supporters warned she could die in prison after suffering two suspected heart attacks earlier this year.
Mohammadi, 54, was awarded the 2023 Nobel Peace Prize for her activism against female oppression in Iran and promoting human rights.
After pleas from her family for her to be transferred from prison, Mohammadi is "now at Tehran Pars Hospital to be treated by her own medical team", the Narges Mohammadi Foundation said in a statement.
She had spent 10 days hospitalised in Zanjan in northern Iran, where she had been serving her sentence.
Mohammadi's Paris-based husband said "she is not in a favourable general condition" and that "her status remains unstable", in a statement over the weekend.
The activist is believed to have lost about 20kg (three stone) while in prison, and has difficulty speaking and is barely recognisable, according to her lawyer Chirinne Ardakani.
In 2021, Mohammadi began serving a 13-year sentence on charges of committing "propaganda activity against the state" and "collusion against state security", which she denied.
In December 2024, she was given a temporary release from Tehran's notorious Evin prison on medical grounds.
Mohammadi was arrested last December for making "provocative remarks" at a memorial ceremony, Iranian authorities said at the time. Her family said she was taken to hospital after being beaten during the arrest.
