Short answer
A Muslim woman may be married to only one man at a time. A second marriage while she is still married is void and counts as zina, by agreement of all the scholars, because the Quran forbids married women to other men. Scholars explain the wisdom as protecting each child's lineage and the clarity of family rights.
A woman who is married, or in the iddah of a previous marriage, is forbidden to every other man. A nikah with her is void (batil). After divorce or her husband's death, and once her iddah ends, she may marry again.
The four schools agree on this.
Explanation
Among the women the Quran lists as forbidden in marriage are "married women". So a woman who is married cannot marry anyone else until that marriage ends and her waiting period (iddah) is complete. This is agreed by all the schools and has never been disputed.
The scholars mention several reasons:
- Lineage (nasab): every child must know who its father is. Inheritance, mahram relationships, custody and maintenance all depend on it. With several husbands, this would be lost.
- The iddah exists for the same reason: to make sure a womb is clear before a new marriage.
- Clear rights: a husband is responsible for his wife's maintenance and the household; that responsibility cannot be split between men.
- Human nature: scholars note that men's jealousy over a wife is strong and natural, and shared marriage would breed conflict.
People often ask why a man may marry up to four. Polygyny comes with strict conditions of justice and maintenance, and most Muslim men have one wife. See the conditions for a second wife. A woman also has rights in marriage that a man does not, such as mahr and full maintenance. See the rights of a wife.
A woman who is divorced or widowed may remarry once her iddah ends. See the iddah calculator.
The evidence
Quran, Surah An-Nisaa 4:24
۞ وَٱلْمُحْصَنَٰتُ مِنَ ٱلنِّسَآءِ إِلَّا مَا مَلَكَتْ أَيْمَٰنُكُمْ ۖ كِتَٰبَ ٱللَّهِ عَلَيْكُمْ ۚ وَأُحِلَّ لَكُم مَّا وَرَآءَ ذَٰلِكُمْ أَن تَبْتَغُوا۟ بِأَمْوَٰلِكُم مُّحْصِنِينَ غَيْرَ مُسَٰفِحِينَ ۚ فَمَا ٱسْتَمْتَعْتُم بِهِۦ مِنْهُنَّ فَـَٔاتُوهُنَّ أُجُورَهُنَّ فَرِيضَةًۭ ۚ وَلَا جُنَاحَ عَلَيْكُمْ فِيمَا تَرَٰضَيْتُم بِهِۦ مِنۢ بَعْدِ ٱلْفَرِيضَةِ ۚ إِنَّ ٱللَّهَ كَانَ عَلِيمًا حَكِيمًۭا ﴿٢٤﴾
And [also prohibited to you are all] married women except those your right hands possess. [This is] the decree of Allah upon you. And lawful to you are [all others] beyond these, [provided] that you seek them [in marriage] with [gifts from] your property, desiring chastity, not unlawful sexual intercourse. So for whatever you enjoy [of marriage] from them, give them their due compensation as an obligation. And there is no blame upon you for what you mutually agree to beyond the obligation. Indeed, Allah is ever Knowing and Wise.
A married woman cannot marry another man.
Quran, Surah Al-Baqara 2:228
وَٱلْمُطَلَّقَٰتُ يَتَرَبَّصْنَ بِأَنفُسِهِنَّ ثَلَٰثَةَ قُرُوٓءٍۢ ۚ وَلَا يَحِلُّ لَهُنَّ أَن يَكْتُمْنَ مَا خَلَقَ ٱللَّهُ فِىٓ أَرْحَامِهِنَّ إِن كُنَّ يُؤْمِنَّ بِٱللَّهِ وَٱلْيَوْمِ ٱلْءَاخِرِ ۚ وَبُعُولَتُهُنَّ أَحَقُّ بِرَدِّهِنَّ فِى ذَٰلِكَ إِنْ أَرَادُوٓا۟ إِصْلَٰحًۭا ۚ وَلَهُنَّ مِثْلُ ٱلَّذِى عَلَيْهِنَّ بِٱلْمَعْرُوفِ ۚ وَلِلرِّجَالِ عَلَيْهِنَّ دَرَجَةٌۭ ۗ وَٱللَّهُ عَزِيزٌ حَكِيمٌ ﴿٢٢٨﴾
Divorced women remain in waiting for three periods, and it is not lawful for them to conceal what Allah has created in their wombs if they believe in Allah and the Last Day. And their husbands have more right to take them back in this [period] if they want reconciliation. And due to the wives is similar to what is expected of them, according to what is reasonable. But the men have a degree over them [in responsibility and authority]. And Allah is Exalted in Might and Wise.
The iddah protects lineage, the same reason a woman has one husband.
Where the ruling is recorded
- al-Hidaya, Kitab al-Nikah, chapter on women forbidden in marriage. A woman married to another man, or in her iddah, cannot be married.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.