The Saudi Arabia supreme court ruled on Tuesday evening that the Islamic calendar month of Shaban, which precedes Ramadan, will end on Wednesday, meaning Ramadan will begin the following day, the official Saudi Press Agency reported.
Authorities earlier on Tuesday called on residents to try to spot the crescent moon that marks the start of Ramadan, but it was not visible, state media said.
The official Crescent Sighting Committee in neighboring Qatar also announced that Thursday "is the first day of the blessed month of Ramadan", the official Qatar News Agency said on Twitter.
The starting date of Ramadan, the holiest Muslim month, is set by both lunar calculations and physical sightings to determine the beginning of a new month.
The daytime fasting month of Ramadan is one of the five pillars of Islam.
Observant Muslims refrain from eating and drinking from dawn to dusk, and traditionally gather with family and friends to break their fast in the evening.
Fasting is strictly observed in Saudi Arabia, with restaurants closed until sunset iftar meals.
It is also a time of prayers, with the faithful converging in large numbers at mosques, especially at night.