Yes, they can do so provided that regular arrangements to insure their veil, privacy, security, moral well-being, education, self-purification and spiritual cleansing are made. Thousands of men and women in many big mosques and Islamic education centres, do sit for al-itikaf-retreat with well-assured segregation, privacy and security. They are given a well-managed timetable for their daily course of deeds. Although we do not argue that fulfilling all the requisites is possible in every mosque.

About the issue of a woman sitting for al-itikaf-retreat in a mosque, Ibn Nujaym al-Hanafi writes:

إِنَّ اعْتِكَافَهَا فِي مَسْجِدِ الْـجَمَاعَةِ جَائِزٌ.

The spiritual retreat of a woman in a mosque of congregation (where the five daily prayers are regularly held) is allowed beyond doubt.

He further writes with quoting al-Kasani from Bada'i' al-Sana'i':

إِنَّ اعْتِكَافَهَا فِي مَسْجِدِ الْجَمَاعَةِ صَحِيْحٌ بِلَا خِلَافٍ بَيْنَ أَصْحَابِنَا.

Her itikaf in the mosque of congregation is undoubtedly valid, without a single opinion amongst our scholars to differ.

Ibn 'Abidin al-Shami, in his book Rad al-Mu'tar (2:441) declares her sitting for spiritual retreat in a mosque al-makruh al-tanzihi. He also quotes Bada'i' al-Sana'i':

صُرِّحَ فِي الْبَدَائِعِ أَنَّهُ خِلَافُ الْأَفْضَلِ.

It is clearly stated in al-Bada'i' that it is against the preferable.

It means that her spiritual retreat at home is preferable to the mosque. Here it is important to mention that not every mosque can make proper arrangements for women’s spiritual retreat and this is the basis of the prohibition against it. A mosque, if spacious enough to accommodate both men and women with all the requisites of segregation, and capable enough to enforce a routine of educational and spiritual purification through a timetable of optional prayers along with the obligations, sending of greetings and salutation to the Prophet a, al-dhikr, admonition, advice and regular invocations, can enhance the prospect of benefit and reward to a far higher degree, as compared to the individual/private activity of spiritual retreat.

The collective spiritual retreat held at the centre of Minhaj-ul-Quran is an example of this, where both male and female attendants from far and near, prosper with spiritual training. Women travel all the way to the place of spiritual retreat with their husbands or some other member of their family such as brothers or fathers or grandfathers etc., where a spacious area adjacent to the mosque is made available for their accommodation and meals in complete segregation from men. In order to serve them there and make their stay as comfortable and easy as possible, hundreds of young female students from Minhaj Girls’ College and senior members of the Minhaj Women’s League, take charge inside the building. Armed guards are also appointed at the doors. All the attendants are registered. A 24/7 presence of lady doctors, paramedics and teams of first aid helpers is charged to deal with illness and accidents. The volunteers also include those responsible for cleanliness in the premises. Regular educational circles for learning the Qur'an, hadith, fiqh and tasawwuf are in place. All this is neither possible nor available at every place. Their importance does not need stressing. We can also refer back to question number 114.