Sotoudeh said she was launching her protest over the recent arrest of a prominent civil rights activist and the harassment of his, and her own, family by security forces.
Sotoudeh served about half of a six-year jail sentence imposed in 2010 for spreading propaganda and conspiring to harm state security - charges she denied - before being freed in 2013.
She was arrested again in June and her husband, Reza Khandan, told Iranian media she is facing security charges.
"Since none of my correspondence with the authorities has produced a result, I have no choice but to start a hunger strike as of [August 25, 2018] in protest against the judicial arrests and pressure against my family, relatives and friends," Sotoudeh wrote on Khandan's Facebook page.
She added the words, "With the hope of the establishment of law and justice in our dear country Iran."
Sotoudeh, who has represented Iranian opposition activists, embarked on an earlier 50-day hunger strike in 2012 against a travel ban on her daughter.
Her case then caused an international outcry in which the United States and the human rights group Amnesty International criticized the Islamic Republic and she was freed in September 2013 ahead of a visit to the United Nations by President Hassan Rouhani, soon after he was swept into office in part on promises of liberal reforms.
Sotoudeh recently represented a number of women who have removed their headscarves, or hijab, in public to protest against Iran’s mandatory Islamic dress code for women, according to the Center for Human Rights in Iran, a New York-based advocacy group.