Short answer
Yes. Sex outside a valid nikah is zina, one of the major sins, whether before marriage or with someone other than your spouse. Allah forbids even approaching it. The four schools agree, and sincere repentance is always open.
Zina is haram by the Quran, the sunnah and consensus, and it is a major sin (kabirah). What leads to it, such as being alone with a non-mahram (khalwah), touching, kissing and sexual talk, is also haram.
The four schools agree on this.
Explanation
Allah does not only forbid zina (sexual intercourse outside marriage). He says, "do not approach" it (17:32), which closes the doors that lead to it. In Surah al-Furqan zina is named next to shirk and murder among the sins that bring a multiplied punishment, unless the person repents (25:68 to 70).
The Prophet ﷺ said that a person is not a believer at the moment he commits zina. The scholars explain that faith leaves him at that moment and returns when he stops and repents. It does not make him a non-Muslim, but it shows how grave the act is. He also said the eyes and the tongue have their own zina, in looking and in speech, which is why the steps before intercourse are also sinful.
- "We plan to marry." An intention to marry does not make sex lawful. Only the nikah does. If you want to be together, do the nikah now. See conditions of a valid nikah.
- Living together. A man and a woman who are not married may not live as a couple.
- Other acts. Kissing, touching and sexual chat are not zina in the legal sense, but they are haram. See kissing before marriage.
- A doubtful marriage. If you are unsure whether your civil marriage or nikah was valid, ask a mufti.
If it has already happened, do not despair and do not tell people. Stop, regret it and turn to Allah. Read repenting from zina, and on the legal punishment, punishment for zina.
The evidence
Quran, Surah Al-Israa 17:32
وَلَا تَقْرَبُوا۟ ٱلزِّنَىٰٓ ۖ إِنَّهُۥ كَانَ فَٰحِشَةًۭ وَسَآءَ سَبِيلًۭا ﴿٣٢﴾
And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way.
Allah forbids zina and everything that brings a person near it.
Quran, Surah Al-Furqaan 25:68-70
وَٱلَّذِينَ لَا يَدْعُونَ مَعَ ٱللَّهِ إِلَٰهًا ءَاخَرَ وَلَا يَقْتُلُونَ ٱلنَّفْسَ ٱلَّتِى حَرَّمَ ٱللَّهُ إِلَّا بِٱلْحَقِّ وَلَا يَزْنُونَ ۚ وَمَن يَفْعَلْ ذَٰلِكَ يَلْقَ أَثَامًۭا ﴿٦٨﴾ يُضَٰعَفْ لَهُ ٱلْعَذَابُ يَوْمَ ٱلْقِيَٰمَةِ وَيَخْلُدْ فِيهِۦ مُهَانًا ﴿٦٩﴾ إِلَّا مَن تَابَ وَءَامَنَ وَعَمِلَ عَمَلًۭا صَٰلِحًۭا فَأُو۟لَٰٓئِكَ يُبَدِّلُ ٱللَّهُ سَيِّـَٔاتِهِمْ حَسَنَٰتٍۢ ۗ وَكَانَ ٱللَّهُ غَفُورًۭا رَّحِيمًۭا ﴿٧٠﴾
And those who do not invoke with Allah another deity or kill the soul which Allah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever should do that will meet a penalty. Multiplied for him is the punishment on the Day of Resurrection, and he will abide therein humiliated - Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah Forgiving and Merciful.
Zina is listed among the gravest sins, with the door of repentance left open.
Sahih al-Bukhari 2475
Sahih... لا يزني الزاني حين يزني وهو مؤمن ...
"The one who commits zina is not a believer at the moment he commits it."
Shows how seriously zina damages a person's faith.
Sahih al-Bukhari 6243
Sahih... فزنا العين النظر وزنا اللسان المنطق ...
"The zina of the eyes is looking, and the zina of the tongue is speech."
The steps that lead to zina are sins too.
Where the ruling is recorded
- al-Hidaya, Kitab al-Hudud, Bab al-Zina. Defines zina and the conditions for its punishment.
- al-Fatawa al-Hindiyya, Kitab al-Karahiyya, chapter on looking and touching. Touching or looking at a non-mahram with desire is haram.
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.