وَلَا تَقْرَبُوا۟ ٱلزِّنَىٰٓ ۖ إِنَّهُۥ كَانَ فَٰحِشَةًۭ وَسَآءَ سَبِيلًۭا ﴿٣٢﴾
你们不要接近私通，因为私通确是下流的事，这行径真恶劣！
Haram Family & relations
Is zina (sex outside marriage) haram?
Short answer
Yes. The Quran says not to approach unlawful sexual intercourse, for it is an immorality and an evil way (17:32). It is among the major sins.
Evidence
Quran
ٱلزَّانِيَةُ وَٱلزَّانِى فَٱجْلِدُوا۟ كُلَّ وَٰحِدٍۢ مِّنْهُمَا مِا۟ئَةَ جَلْدَةٍۢ ۖ وَلَا تَأْخُذْكُم بِهِمَا رَأْفَةٌۭ فِى دِينِ ٱللَّهِ إِن كُنتُمْ تُؤْمِنُونَ بِٱللَّهِ وَٱلْيَوْمِ ٱلْءَاخِرِ ۖ وَلْيَشْهَدْ عَذَابَهُمَا طَآئِفَةٌۭ مِّنَ ٱلْمُؤْمِنِينَ ﴿٢﴾
淫妇和奸夫，你们应当各打一百鞭。 你们不要为怜悯他俩而减免真主的刑罚，如果你们确信真主和末日。叫一伙信士，监视他俩的受刑。
Hadith
Sahih al-Bukhari 2475
Narrated Abu Huraira:The Prophet (ﷺ) said, "When an adulterer commits illegal sexual intercourse, then he is not a believer at the time, he is doing it, and when a drinker of an alcoholic liquor drinks it, then he is not a believer at the time of drinking it, and when a thief steals, then he is not a believer at the time of stealing, and when a robber robs, and the people look at him, then he is not a believer at the time of doing robbery
Arabic text (with chain of narrators)
حَدَّثَنَا سَعِيدُ بْنُ عُفَيْرٍ، قَالَ حَدَّثَنِي اللَّيْثُ، حَدَّثَنَا عُقَيْلٌ، عَنِ ابْنِ شِهَابٍ، عَنْ أَبِي بَكْرِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ قَالَ النَّبِيُّ صلى الله عليه وسلم " لاَ يَزْنِي الزَّانِي حِينَ يَزْنِي وَهْوَ مُؤْمِنٌ، وَلاَ يَشْرَبُ الْخَمْرَ حِينَ يَشْرَبُ وَهْوَ مُؤْمِنٌ، وَلاَ يَسْرِقُ حِينَ يَسْرِقُ وَهْوَ مُؤْمِنٌ، وَلاَ يَنْتَهِبُ نُهْبَةً يَرْفَعُ النَّاسُ إِلَيْهِ فِيهَا أَبْصَارَهُمْ حِينَ يَنْتَهِبُهَا وَهْوَ مُؤْمِنٌ ". وَعَنْ سَعِيدٍ وَأَبِي سَلَمَةَ عَنْ أَبِي هُرَيْرَةَ عَنِ النَّبِيِّ صلى الله عليه وسلم مِثْلَهُ إِلاَّ النُّهْبَةَ.
This is general information, not a fatwa. For your own situation, ask a qualified scholar.
Questions about halal and haram
Is zina (sex outside marriage) haram?
Yes. The Quran says not to approach unlawful sexual intercourse, for it is an immorality and an evil way (17:32). It is among the major sins.