The Narges Foundation said Ms Mohammadi, 53, was detained in the eastern city of Mashhad, along with other activists.
The Nobel Committee said it was "deeply concerned by today's brutal arrest of Narges Mohammadi" and called on the authorities "to immediately clarify Mohammadi's whereabouts, ensure her safety and integrity, and to release her without conditions". Iran does not appear to have commented.
Ms Mohammadi was awarded the 2023 Nobel Peace Prize for her activism against female oppression in Iran and promoting human rights.
In December 2024, she was given a temporary release from jail for three weeks on medical grounds, after being held in Tehran's notorious Evin prison since 2021.
She was expected to return to prison soon after, where she was serving multiple sentences.
Her latest arrest reportedly came as she attended a memorial ceremony for Khosrow Alikordi, a lawyer found dead in his office last week.
Norway-based group Iran Human Rights had called for an independent inquiry to establish how he died, saying the circumstances around his death were "suspicious".
Several other activists were detained at the ceremony, where it's reported they shouted slogans, including "death to the dictator" and "long live Iran".
Taghi Rahmani, Ms Mohammadi's husband, told BBC Persian: "They arrested Narges violently. The brother of the lawyer witnessed her arrest at the memorial.
"This act is against human rights laws, and amounts to some kind of revenge.
"This happened in Mashhad today and is concerning because the establishment's crackdown has intensified recently."
