Short answer
No. Straightening, curling, perming and relaxing change the style of your own hair for a time, which is allowed adornment. Men should avoid styles that copy women, everyone should avoid harmful products, and anything that leaves a coating on the hair must be washed out before ghusl.
Styling one's own hair with heat or chemical treatments is permissible (mubah) for women, and for men as long as the style is not specific to women. Products that harm the scalp or health should be avoided.
The four schools agree on this.
Explanation
Allah asks who has forbidden the adornment He brought out for His servants, and the Prophet ﷺ said Allah is beautiful and loves beauty. He also told those who have hair to honour it. Looking after your hair and styling it is part of this, not vanity.
The forbidden changes to the body are named in the hadith: tattooing, plucking the eyebrows, filing the teeth and joining hair. Straightening or curling your own hair is none of these. Like dyeing it, the effect grows out, and no part of the body is cut or added to, so the scholars treat it as ordinary adornment.
- Heat styling, curling and perms: allowed.
- Relaxers and keratin or smoothing treatments: allowed in themselves, but some products carry real health risks. Ask about the ingredients and avoid what is harmful.
- Men: curls, waves or a straightened look are fine, but not a style that belongs to women in your culture; see which haircuts are haram.
- Women: show the result only to your husband, mahrams and other women; see going to a hair salon.
Ghusl and keratin. For ghusl, water must reach the scalp and, for loose hair, every hair; a woman whose hair is braided only needs water to reach the roots. Heat styling, perms and relaxers change the hair itself and leave no layer, so ghusl is valid. Keratin and smoothing treatments are the open question: if the product soaks into the hair and leaves no film, ghusl is valid; if it seals the hair with a layer water cannot pass through, like a wax or coating spray, that layer must be washed out first. Salons often say not to wash for three days after keratin; if ghusl becomes due, ghusl comes first. The same rule covers nail polish and anything else that blocks water; see wudu with nail polish and how to do ghusl.
Ask a mufti about your own case. This answer explains the general ruling. When the details of your situation matter, as they do here, ask a qualified mufti and give them the full facts. Where to ask a scholar.
The evidence
Quran, Surah Al-A'raaf 7:32
قُلْ مَنْ حَرَّمَ زِينَةَ ٱللَّهِ ٱلَّتِىٓ أَخْرَجَ لِعِبَادِهِۦ وَٱلطَّيِّبَٰتِ مِنَ ٱلرِّزْقِ ۚ قُلْ هِىَ لِلَّذِينَ ءَامَنُوا۟ فِى ٱلْحَيَوٰةِ ٱلدُّنْيَا خَالِصَةًۭ يَوْمَ ٱلْقِيَٰمَةِ ۗ كَذَٰلِكَ نُفَصِّلُ ٱلْءَايَٰتِ لِقَوْمٍۢ يَعْلَمُونَ ﴿٣٢﴾
Say, "Who has forbidden the adornment of Allah which He has produced for His servants and the good [lawful] things of provision?" Say, "They are for those who believe during the worldly life [but] exclusively for them on the Day of Resurrection." Thus do We detail the verses for a people who know.
Adornment and good things are lawful unless Allah has forbidden them.
Sahih Muslim 91
Sahih... إن الله جميل يحب الجمال ...
"Allah is beautiful and loves beauty."
Said to a man who asked whether liking good clothes and shoes was pride; it shows caring for one's appearance is not blameworthy.
Sunan Abi Dawud 4163
Graded hasan sahih... من كان له شعر فليكرمه ...
"Whoever has hair, let him honour it."
Caring for the hair is encouraged.
Sunan Ibn Majah 2340
Graded sahih... لا ضرر ولا ضرار ...
"There is to be no harm and no returning of harm."
The general rule that harming yourself or others is not allowed.
Where the ruling is recorded
- al-Durr al-Mukhtar with Radd al-Muhtar, Kitab al-Hazr wa al-Ibaha, discussion of adornment and joining hair. Which changes to the hair are forbidden and which are adornment.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Taharah, obligations of ghusl. Water must reach every hair and the scalp.
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.