Short answer
Yes. In Islam an adopted child must know who their real parents are, as far as you know, because lineage, marriage, mahram and inheritance all depend on it. Hiding it so the child believes you are the birth parents is not allowed. Tell them early, gently and in words that suit their age.
Concealing a child's lineage so that they are taken as your biological child is haram, since it ascribes them to someone other than their father. Making the truth known to the child and keeping records of the birth family is required in effect, because the rulings of nikah, mahram and inheritance rest on it.
The four schools agree on this.
Explanation
The Quran tells us to call children by their fathers' names, and the Prophet ﷺ warned against claiming a father who is not your own. A child raised to believe their adoptive parents are their birth parents is being ascribed to someone else, even if no one says it in words. The four schools agree this is not permitted.
Why it matters so much:
- Marriage: the child may one day want to marry a birth relative without knowing, or may think an adoptive sibling is forbidden when they are not.
- Mahram and hijab: adoptive parents and siblings of the opposite sex are not mahram unless there is a milk tie. See is an adopted child mahram.
- Inheritance: the child inherits from the birth family, not as an heir of the adoptive family.
- Trust: many adoptees who learn the truth late feel deceived. Honesty from the start protects the relationship.
How to tell them:
- Start early, with simple words: "You grew in another lady's tummy, and Allah brought you to us." Build on it as they grow.
- Speak of the birth parents with respect, whatever the circumstances.
- Keep their birth name, documents and any family details safe for them.
- Use Islam's own language: the Prophet ﷺ himself raised Zayd in his home and loved him dearly.
- Get support from your adoption or fostering agency, which will usually recommend openness too.
If the birth father is unknown, the child is still your brother or sister in faith and in your care, as the same ayah says. See the adopted child's surname and adoption in Islam.
The evidence
Quran, Surah Al-Ahzaab 33:5
ٱدْعُوهُمْ لِءَابَآئِهِمْ هُوَ أَقْسَطُ عِندَ ٱللَّهِ ۚ فَإِن لَّمْ تَعْلَمُوٓا۟ ءَابَآءَهُمْ فَإِخْوَٰنُكُمْ فِى ٱلدِّينِ وَمَوَٰلِيكُمْ ۚ وَلَيْسَ عَلَيْكُمْ جُنَاحٌۭ فِيمَآ أَخْطَأْتُم بِهِۦ وَلَٰكِن مَّا تَعَمَّدَتْ قُلُوبُكُمْ ۚ وَكَانَ ٱللَّهُ غَفُورًۭا رَّحِيمًا ﴿٥﴾
Call them by [the names of] their fathers; it is more just in the sight of Allah. But if you do not know their fathers - then they are [still] your brothers in religion and those entrusted to you. And there is no blame upon you for that in which you have erred but [only for] what your hearts intended. And ever is Allah Forgiving and Merciful.
Lineage must be kept, and a child of unknown father is still a brother in faith.
Sahih al-Bukhari 6766
Sahih... من ادعى إلى غير أبيه وهو يعلم أنه غير أبيه ...
"Whoever claims a father other than his own, knowing he is not his father..."
Ascribing a child to someone other than the father is forbidden.
Where the ruling is recorded
- al-Hidaya, Kitab al-Laqit (the foundling). The foundling's lineage and the carer's role.
- Ahkam al-Quran (al-Jassas), Commentary on Surat al-Ahzab, ayah 5. A Hanafi explanation of the ban on ascribing children to other than their fathers.
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.