Seema Ali Osman, from the Nuba Mountains was taken by police in June along with a number of other women. She's the last to face trial.
Last week a court found four women not guilty while three other were fined 50 Sudense pounds (approximately £5).
In July, two others were fined 500 Sudanese pounds (approximately £50).
Critics say the law is too broad and allows police too much scope to define what constitutes indecent or immoral dress.
Mervyn Thomas, Chief Executive of Christian Solidarity Worldwide said, "We welcome Seema Ali Osman's release but continue to question the inconsistent handling of these women's cases, which appear to be part of an ongoing campaign of repression against ethnic and religious minorities in Sudan.
"We remain deeply concerned by the ambiguity and arbitrary application of the law, and renew our call for it to be clarified or repealed.
"The African Union and the rest of the international community must hold Sudan to account for its failure to respect the right to freedom of religion or belief, as outlined in Article 18 of the International Covenant on Civil and Political Rights (ICCPR), to which Sudan is party."