Short answer
Islam allows a man up to four wives, on the condition of justice, to meet real needs: widows and orphans, more women than men after wars, a wife's long illness or infertility, and a man's need for a lawful bond rather than a hidden affair. It is a permission, not a duty, and a man who fears being unjust is commanded to marry only one.
Marrying more than one wife is permissible (mubah) for a man who can support each wife and treat them equally in nights, housing and provision. It is not a sunnah in itself, and if he fears he will be unjust, keeping to one wife is wajib on him.
The four schools agree on this.
Explanation
Before Islam, Arab men married as many women as they liked, with no rights attached. The Quran limited this to four and tied it to justice: "marry those that please you of women, two or three or four. But if you fear that you will not be just, then marry only one" (4:3). When Ghaylan al-Thaqafi became Muslim with ten wives, he was told to keep four. So Islam did not invent polygamy. It restricted it.
The ayah begins with orphans: Aisha explained that it was about guardians who wanted to marry orphan girls in their care without giving them a fair mahr. Beyond that, scholars point to needs like these:
- Widows and divorced women who want a husband and support.
- Times when women outnumber marriageable men.
- A wife's long illness, or a couple's infertility, where the husband does not want to divorce her.
- A man with strong desire who would otherwise fall into zina. Islam prefers an open, responsible marriage to a secret affair that leaves a woman with no rights.
The permission comes with heavy duties: a separate home for each wife, equal nights and fair provision. The Prophet ﷺ warned that a man who leans towards one wife will come on the Day of Judgement with one side of his body leaning. Allah also says that perfect equality of feeling is impossible, but a husband must never leave a wife "hanging" (4:129). See the conditions for a second wife.
Most Muslim men have always had one wife. The Prophet ﷺ was married to Khadijah alone for about twenty-five years. Where the law of the land bans a second marriage, the second wife has no legal protection, which scholars take seriously. On why a woman may not have several husbands, see that question.
The evidence
Quran, Surah An-Nisaa 4:3
وَإِنْ خِفْتُمْ أَلَّا تُقْسِطُوا۟ فِى ٱلْيَتَٰمَىٰ فَٱنكِحُوا۟ مَا طَابَ لَكُم مِّنَ ٱلنِّسَآءِ مَثْنَىٰ وَثُلَٰثَ وَرُبَٰعَ ۖ فَإِنْ خِفْتُمْ أَلَّا تَعْدِلُوا۟ فَوَٰحِدَةً أَوْ مَا مَلَكَتْ أَيْمَٰنُكُمْ ۚ ذَٰلِكَ أَدْنَىٰٓ أَلَّا تَعُولُوا۟ ﴿٣﴾
And if you fear that you will not deal justly with the orphan girls, then marry those that please you of [other] women, two or three or four. But if you fear that you will not be just, then [marry only] one or those your right hand possesses. That is more suitable that you may not incline [to injustice].
The permission and its condition of justice.
Quran, Surah An-Nisaa 4:129
وَلَن تَسْتَطِيعُوٓا۟ أَن تَعْدِلُوا۟ بَيْنَ ٱلنِّسَآءِ وَلَوْ حَرَصْتُمْ ۖ فَلَا تَمِيلُوا۟ كُلَّ ٱلْمَيْلِ فَتَذَرُوهَا كَٱلْمُعَلَّقَةِ ۚ وَإِن تُصْلِحُوا۟ وَتَتَّقُوا۟ فَإِنَّ ٱللَّهَ كَانَ غَفُورًۭا رَّحِيمًۭا ﴿١٢٩﴾
And you will never be able to be equal [in feeling] between wives, even if you should strive [to do so]. So do not incline completely [toward one] and leave another hanging. And if you amend [your affairs] and fear Allah - then indeed, Allah is ever Forgiving and Merciful.
A husband must not neglect any wife.
Sunan Abi Dawud 2133
Graded sahih... من كانت له امرأتان فمال إلى إحداهما جاء يوم القيامة وشقه مائل ...
"Whoever has two wives and inclines to one of them will come on the Day of Resurrection with one side of his body leaning."
Injustice between wives is a serious sin.
Where the ruling is recorded
- al-Hidaya, Kitab al-Nikah, Bab al-Qasm. Equal division of nights and provision between wives.
- Bada'i al-Sana'i (al-Kasani), Kitab al-Nikah, on the number of wives. The limit of four and the condition of justice.
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.