"Supreme Court declares tomorrow the last day of #Ramadan and Wednesday the first day of #Eid Al-Fitr," the official Saudi Press Agency said on its X account.
The timing of Eid al-Fitr is determined by the sighting of the crescent moon, in accordance with the Muslim lunar calendar.
Saudi media outlets reported that the crescent moon was not visible on Monday.
The United Arab Emirates and Qatar also announced that Eid al-Fitr, a holiday normally celebrated with family gatherings, would begin on Wednesday.
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.
Ramadan is also a time for prayer, with the faithful converging in large numbers on mosques, especially at night.
Fasting is widely practised in Saudi Arabia, home of the Grand Mosque in Mecca and the Prophet's Mosque in Medina.
Saudis are expected to observe a four-day holiday for Eid al-Fitr.
Across the Muslim world, Ramadan festivities this year were overshadowed by the war in Gaza, where Israel's military campaign to eradicate Hamas has killed at least 33,207 people, mostly women and children, according to the health ministry in the Hamas-run territory.
The war was sparked by the October 7 attack against Israel by Hamas militants that resulted in the deaths of 1,170 people, mostly civilians, Israeli figures show.
Palestinian militants also took more than 250 Israeli and foreign hostages, 129 of whom remain in Gaza, including 34 the army says are dead.
Mediators pushed in vain for a cease-fire to take effect before the start of Ramadan.
Talks on a cease-fire have resumed in Cairo, but no breakthrough has yet been announced.