She was referring to the avenue where the woman took off her headscarf - a punishable offence in Iran.
The lawyer said she had gone to the prosecutor's office to follow up the woman's case and had learned of her release the previous day.
"I hope they don't fabricate a legal case to harm her for using her basic rights," Mrs Sotoudeh wrote. "She has not done anything wrong to deserve prosecution."
The woman is believed to be the 31-year-old mother of a toddler.
How have Iranians reacted?
The woman has been the subject of a social media campaign in Iran following the anti-establishment protests at the end of last year in which at least 20 people died.
A hashtag in Persian asking about her whereabouts - and English-language equivalents #where_is_she and #WhereIsShe - have been used thousands of times on Twitter, as well as on other social media channels used in the country where dissent is often met with repression.