Are fatwas issued by women valid? Who are the women who were known for  knowledge and issuing fatwas in Islamic history?
Question

Is it valid for women to issue fatwas?
Praise be to Allah.Allah, may He be exalted, sent His Messenger with guidance and the religion of truth, to be a bringer of glad tidings and a warner to all people. The Prophet (blessings and peace of Allah be upon him) fulfilled the trust and conveyed the message. Among those who heard him (blessings and peace of Allah be upon him) were both women and men. So those who undertook the mission of conveying and teaching Islam were the Sahaabah and those who came after them; their students included both women and men, and the process of learning and teaching was done in accordance with Islamic guidelines, far removed from any haraam mixing, or women uncovering their faces, or speaking softly.
Islam does not differentiate between men and women with regard to knowledge and issuing fatwas. The scholars have stipulated the conditions to be met by the mufti (scholar who issues fatwas), and they did not mention among these conditions that the mufti must be male. No difference of scholarly opinion is known concerning that.
An-Nawawi (may Allah have mercy on him) said:
The conditions to be met by the mufti are that he should be accountable, Muslim, trustworthy, honest, far removed from evildoing and from actions that could cast aspersions on his [or her] integrity, have an innate ability to grasp the meanings of things, be clear-minded and mature in thinking, of sound understanding and able to reach sound conclusions, and alert. This applies equally to one who is free, a slave, a woman, one who is blind, and one who is nonverbal, if he or she can write or his or her gestures can be understood. End quote. Al-Majmoo‘ Sharh al-Muhadhdhab (1/41).
Among this great ummah there were women who undertook the duty of teaching and issuing fatwas, just as men did.
Yes, the number of women cannot be compared to that of men, because of the nature of man, but this does not mean that there were not women who undertook the mission of teaching and issuing fatwas. Foremost among those women was the Mother of the Believers ‘Aa’ishah (may Allah be pleased with her). Both men and women would seek her out to ask for fatwas, because she had been so close to the Prophet (blessings and peace of Allah be upon him), and because of her intelligence and smartness.
Ibn ‘Abd al-Barr said in al-Isti‘aab (p. 609):
It was narrated that Masrooq said: I saw some of the elderly senior companions of the Messenger of Allah (blessings and peace of Allah be upon him) asking her about issues pertaining to the shares of inheritance.
‘Ata’ ibn Abi Rabaah said: ‘Aa’ishah was the most knowledgeable of the people and had the greatest understanding of religion.
Hishaam ibn ‘Urwah said, narrating from his father: I never saw anyone who was more knowledgeable of fiqh, medicine or poetry than ‘Aa’ishah.
Az-Zuhri said: If the knowledge of ‘Aa’ishah was compared to the knowledge of all the other wives of the Prophet (blessings and peace of Allah be upon him) and the knowledge of all women put together, the knowledge of ‘Aa’ishah would surpass them all. End quote.
Ibn al-Qayyim (may Allah have mercy on him) listed those of the Sahaabah who issued many fatwas, among whom he mentioned ‘Aa’ishah (may Allah be pleased with her). He listed Umm Salamah (may Allah be pleased with her) as one of those who issued a middling number of fatwas; and he listed Umm ‘Atiyyah, Safiyyah the Mother of the Believers, Hafsah and Umm Habeebah as being among those who issued a small number of fatwas.
See: I‘laam al-Muwaqqi‘een (1/12, 13).
We will list here some of the women whom Imam adh-Dhahabi (may Allah have mercy on him) described as having knowledge and understanding of religion in his book Siyar A‘laam an-Nubala’, in confirmation of what we have mentioned above.
1.. Safiyyah bint Shaybah.
Adh-Dhahabi (may Allah have mercy on him) said:
Safiyyah bint Shaybah ibn ‘Uthmaan ibn Abi Talhah, the faqeehah and ‘aalimah (scholar).
Siyar A‘laam an-Nubala’ (3/507, 508).
2.. Umm ad-Darda’.
Adh-Dhahabi (may Allah have mercy on him) said:
Umm ad-Darda’, the lady, the ‘aalimah, the faqeehah. She is Umm ad-Darda’ as-Sughra (the Younger).
She narrated a great deal of knowledge from her husband Abu’d-Darda’, and from Salmaan al-Faarisi, Ka‘b ibn ‘Aasim al-Ash‘ari, ‘Aa’ishah, Abu Hurayrah and a number of others.
She recited the entire Qur’an to Abu’d-Darda’ when she was very young.
She lived a long life, and was famous for her knowledge, good deeds and asceticism (zuhd).
Mak-hool said: Umm ad-Darda’ was a faqeehah.
Siyar A‘laam an-Nubala’ (4/277, 278).
3.. Mu‘aadhah al-‘Adawiyyah (d. 83 AH).
Adh-Dhahabi (may Allah have mercy on him) said:
Mu‘aadhah bint ‘Abdillah. The lady, ‘aalimah, mother of as-Sahba’ al-‘Adawiyyah al-Basriyyah, devoted worshipper, wife of the leading example Silah ibn Ashyam.
She narrated from ‘Ali ibn Abi Taalib, ‘Aa’ishah and Hishaam ibn ‘Aamir.
Siyar A‘laam an-Nubala’ (4/508, 509).
4.. Bint al-Mihaamili (d. 377 AH).
Adh-Dhahabi (may Allah have mercy on him) said:
The scholar, faqeehah, mufti, Amat al-Waahid bint al-Husayn ibn Ismaa‘eel.
She was taught by her father and narrated from him, and also from Ismaa‘eel al-Warraaq and ‘Abd al-Ghaafir al-Homsi. She memorized the Qur’an and the fiqh of ash-Shaafa‘i. She was well versed in shares of inheritance, Arabic language, and other subjects.
Al-Burqaani said: She used to issue fatwas with Abu ‘Ali ibn Abi Hurayrah.
Someone else said: She was one of the most well versed of people in fiqh.
Siyar A‘laam an-Nubala’ (15/264).
5.. Kareemah bint Ahmad ibn Muhammad ibn Haatim Umm al-Kiraam al-Marzawiyyah (d. 463 AH).
Adh-Dhahabi (may Allah have mercy on him) said:
The shaykhah, ‘aalimah, woman of virtue, narrator of hadith, Umm al-Kiraam.
Siyar A‘laam an-Nubala’ (18/233, 234).
6.. ‘Aa’ishah bint Hasan bin Ibraaheem (d. 460 AH).
Adh-Dhahabi (may Allah have mercy on him) said:
The preacher, ‘aalimah, narrator of hadith, Umm al-Fath al-Asbahaaniyyah al-Warkaaniyyah.
Ibn as-Sam‘aani said: I asked al-Haafiz Ismaa‘eel about her, and he said: A righteous woman, a scholar who exhorts the women.
Siyar A‘laam an-Nubala’ (18/302).
7. Faatimah ad-Daqqaaq (d. 480 AH).
Adh-Dhahabi (may Allah have mercy on him) said:
Faatimah bint al-Ustaadh az-Zaahid Abu ‘Ali, al-Hasan ibn ‘Ali ad-Daqqaaq, the shaykhah, devoted worshipper, ‘aalimah, Umm al-Baneen an-Naysabooriyyah, wife of al-Ustadh Abu’l-Qaasim al -Qushayri and mother of his children.
Siyar A‘laam an-Nubala’ (18/479, 480).
8. Bint Za‘bah (d. 532 AH).
Adh-Dhahabi (may Allah have mercy on him) said:
The shaykhah, ‘aalimah, reciter of Qur’an, righteous woman, long-lived, the famous hadith narrator of  Naysaboor (Nishapur), Umm al-Khayr, Faatimah bint ‘Ali ibn Muzaffar ibn al-Hasan ibn Za‘bal ibn ‘Ijlaan al-Baghdaadi thumma an-Naysabooriyyah.
Siyar A‘laam an-Nubala’ (19/625).
9.. Faatimah bint al-Baghdaadi (d. 539 AH).
Adh-Dhahabi (may Allah have mercy on him) said:
The shaykhah, ‘aalimah, preacher, righteous woman, long-lived, the famous hadith narrator of Isfahan, Umm al-Baha’, Faatimah bint Muhammad ibn Abi Sa‘d Ahmad ibn al-Hasan ibn ‘Ali ibn al-Baghdaadi al-Asbahaani.
Siyar A‘laam an-Nubala’ (20/148).
This is a list of virtuous women who were scholars and muftis. Islam does not prevent any person of knowledge, man or woman, from issuing fatwas and teaching people the religion of Allah, may He be exalted.
We should point out that there is a difference between being a mufti and being a qaadi (judge). Issuing a fatwa is informing people of the sh ar ‘i ruling, and no one is to be prevented from doing that if he or she is qualified to do it. As for being a qaadi, the evidence of the religious texts, and the practice of the Muslim ummah, indicate that a woman cannot be appointed as a qaadi. This has been explained previously in the answer to question no. 71338 .
And Allah knows best.