For many, this year's event was marred by Israel's war in Gaza.
Saudi authorities discovered the crescent moon and declared Ramadan, a holy month of fasting for many of the world's 1.8 billion Muslims.
Officials observed the moon on Sunday night, marking Monday the first day of the fasting month, Saudi state television said.
This month consists of Muslims abstaining from food and water from sunrise to sunset, reflecting more deeply on their faith, and holding family gatherings. For many, this year's event was marred by Israel's war in Gaza.
After officials in Sunni-majority Saudi Arabia discovered the moon, many Gulf Arab states, Iraq, Syria and Egypt followed suit, confirming their countries would begin fasting from Monday.
However, some Asia-Pacific countries, including Australia, Brunei, Indonesia, Malaysia and Singapore, announced that they would start Ramadan on Tuesday because the crescent moon was not visible.
Oman, on the eastern tip of the Arabian Peninsula, similarly announced that Ramadan would begin on Tuesday. Ramadan begins on Tuesday in Jordan.
Ramadan is celebrated according to the lunar calendar. Additionally, the method of observing the moon often differs from country to country, with some countries declaring the start of the month earlier or later.
Saudi Arabia's King Salman specifically mentioned the Gaza war in his comments after declaring Ramadan.
“While it is heartbreaking to usher in the month of Ramadan this year, given the attacks our Palestinian brethren have suffered, we believe that the international community has a responsibility to stop these heinous crimes and ensure safe and secure We emphasize the need to provide humanitarian assistance 'and relief corridors,''' the King said.
Saudi Arabia had urged its citizens to watch the sky starting Sunday night in preparation for sightings of a crescent moon.
In Iran, which considers itself the world leader of Shiite Muslims, authorities usually begin Ramadan the day after the Sunni one. Iran's Supreme Leader Ayatollah Khamenei's office announced that Ramadan will begin in the country on Tuesday, according to state news agency IRNA.
During Ramadan, observers eat a pre-dawn meal, or “suhoor,” to maintain their strength during the day, and then break their fast with “iftar,” often a hearty meal.
During this month, Muslims try to avoid conflict and focus on charity. However, Israel's war in the Gaza Strip is a major problem for many Muslims. There were hopes that a cease-fire agreement could be reached between Israel and the Palestinian organization Hamas before the start of Ramadan.
More than 31,000 people have been killed in Israel's attack on Gaza, according to Palestinian health officials in the besieged area, and aid agencies have warned that famine is looming in parts of the strip.
Hassna Tabib Hasnan, a dentist who fled the besieged Gaza City in the northern part of the Strip, told AFP news agency: [we] But unfortunately, it is clear that we will live in displacement, suffering, and oppression. ”
Meanwhile, Israeli restrictions on Muslims praying at Jerusalem's al-Aqsa Mosque, Islam's third holiest site, could also further increase tensions in the region.