The annual fasting month of Ramadan is due to begin Monday in Saudi Arabia, Qatar, the United Arab Emirates and Yemen, religious authorities in the four Muslim countries said.
The announcement was made on Sunday after the sighting of the crescent moon.
Other Muslim countries in the Middle East and North Africa are also expected to begin observing Ramadan on Monday or on Tuesday.
During Ramadan, Muslims around the world abstain from eating, drinking, smoking and having sex from dawn to dusk.
The break the fast with a meal known as iftar and before dawn they have a second opportunity to eat and drink during suhur.
Ramadan is sacred to Muslims because tradition says the Koran was revealed to the Prophet Mohammed during that month.
Ramadan is followed by the Eid al-Fitr festival.