In the name of God, the Beneficent, Most Merciful
All praise is due to God (ﷻ), the Lord of the Worlds. The Beneficent, the Merciful. Master of the Day of Judgement, and God’s Peace and Blessings be upon Muhammad (ﷺ), his family, his noble Companions, and all those who follow them.
Your question: Under what conditions can a Muslim woman wears bikini?
Answer: You can but there are conditions on wearing bikini but this must be done while others not looking at you and you must cover 100% your body, if you want to go to the beach where is everyone there because, like this it’s not nice. Just think a little bit, there are boys who likes to see women wearing bikini while not having body fully covered and that will be a temptation. Islam enjoins women to stay at home and not go out unnecessarily, so as to preserve their chastity, maintain their dignity and protect them from all evil. But, you can do this also at home, wearing bikini as you please while you’re alone or you can do this in front of your husband while no one can see you, according to this Hadith:
Ja‘far ibn Muhammad reported that Ahmad was asked about a woman wearing transparent and revealing clothes indoors in front of her husband and he (Ahmad) said, ‘There is no harm in that.’ Ja‘far added, ‘I asked him (Ahmad), 'What if she goes out of her house into the yard unveiled while there is nobody else besides her and her husband in the yard?’ Ahmad said, '[There is] no harm in that.” [Al-Mughni]
Abu Dawood and at-Tirmidhi, who classed the report as hasan, narrated from Abu Maleeh al-Hudhali that some women from Syria entered upon ‘Aa’ishah and she said: Perhaps you are the people whose women enter bathhouses? They said: Yes. She said: I heard the Messenger of God (blessings and peace of God be upon him) say: “There is no woman who removes her clothes anywhere other than her house, but she has torn (the screen) that is between her and God.”
At-Tirmidhi also narrated from Jaabir (may God be pleased with him) that the Prophet (blessings and peace of God be upon him) said: “Whoever believes in God and the Last Day should not let his wife enter bathhouses.” Classed as hasan by al-Albaani in Saheeh at-Tirmidhi.
This is an example for you to understand that also by going to the beach wearing bikini while not having 100% your body covered is a sin according to the Hadiths above. And also, God wants for Muslim women to be dressed modestly.
”And tell the believing women to reduce [some] of their vision and guard their private parts and not expose their adornment except that which [necessarily] appears thereof and to wrap [a portion of] their headcovers over their chests and not expose their adornment except to their husbands, their fathers, their husbands' fathers, their sons, their husbands' sons, their brothers, their brothers' sons, their sisters' sons, their women, that which their right hands possess, or those male attendants having no physical desire, or children who are not yet aware of the private aspects of women. And let them not stamp their feet to make known what they conceal of their adornment. And turn to Allah in repentance, all of you, O believers, that you might succeed.” (An-Nūr 31)
”O Children of Adam, We have brought down to you garments to cover your private parts, as well as for adornment, yet the garment of reverence is the best. These are some of God's signs, perhaps they will take heed.” (7:26)
My advice is try to wear burkini and see how it is, because I saw on the internet many beautiful models of burkini and you can choose anyone of these models and, go ahead and swim.
- Is bathing/swimming without covering the whole body permitted for women?