Short answer
Yes. A non-Muslim may fast alongside Muslim friends, and many do it out of solidarity or curiosity. Fasting is not required of them, and in Islamic belief it counts as worship only with faith. Non-Muslims are warmly welcome at iftar, the meal at sunset, and Muslims love to host them.
Fasting Ramadan is fard only on Muslims, and Islam is a condition for the fast to be valid as worship. A non-Muslim who fasts does nothing wrong. If he accepts Islam during Ramadan, he fasts the remaining days, does not make up the days before, and should hold back from food for the rest of the day he accepted Islam.
The four schools agree on this.
Explanation
The Ramadan fast runs from dawn to sunset: no food, no drink (not even water), no smoking and no intimacy. Muslims fast to grow in taqwa, God-consciousness, as the Quran says. See why Muslims fast.
A non-Muslim can try a day or more. Many colleagues and classmates do, and Muslims appreciate the gesture. Drink plenty at the pre-dawn meal, and do not fast if you have a health condition. Muslims believe the fast is an act of worship accepted with faith, so they will see your fast as kindness and friendship rather than as worship.
- Iftar: the fast is broken at sunset, often with dates and water, then a meal. Mosques and Muslim families often invite neighbours; you do not need to have fasted to come.
- Greeting: Ramadan Mubarak or Ramadan Kareem. See Ramadan greetings.
- At work: you do not need to hide your lunch. Thoughtful touches, like not scheduling team lunches in Ramadan, are appreciated.
If fasting with friends leads you to accept Islam during Ramadan, you hold back from food for the rest of that day, fast as a Muslim from the next day, and do not make up the earlier days. See making up missed fasts and how a non-Muslim can learn about Islam.
The evidence
Quran, Surah Al-Baqara 2:183
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ كُتِبَ عَلَيْكُمُ ٱلصِّيَامُ كَمَا كُتِبَ عَلَى ٱلَّذِينَ مِن قَبْلِكُمْ لَعَلَّكُمْ تَتَّقُونَ ﴿١٨٣﴾
O you who have believed, decreed upon you is fasting as it was decreed upon those before you that you may become righteous -
Fasting is prescribed for believers, as it was for earlier communities.
Jami' at-Tirmidhi 807
Grading to be checked... من فطر صائما كان له مثل أجره ...
"Whoever gives a fasting person food to break his fast will have the same reward as him."
Muslims are eager to host iftar.
Where the ruling is recorded
- al-Hidaya, Kitab al-Sawm, the section on those who become obliged to fast during the day. A non-Muslim who accepts Islam during Ramadan holds back for the rest of the day and makes up nothing before.
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.