Islam is the state religion of Saudi Arabia. As the "home of Islam" where the prophet of Islam lived and carried out his mission, the kingdom attracts millions of Muslim Hajj pilgrims annually, and thousands of clerics and students who come from across the Muslim world to study. Approximately 100% of its citizens are Muslim and most of its large population of foreign workers are as well. Hanbali is the official version of Sunni Islam and it is used in the legal and education systems. The Basic Law of Saudi Arabia states that it is the duty of every citizen to defend Islam.

Religion in Saudi Arabia has had a reach beyond its borders as since the 1970s the Saudi government has spent tens of billions of dollars of its petroleum export revenue throughout the Islamic world and elsewhere promoting Islam and specifically the strict revivalist Salafi school based on the teachings of Muhammad ibn Abd al-Wahhab. However in 2017, Crown Prince Mohammed bin Salman, eliminated many of Wahhabi restrictions (bans on amusement parks, cinemas, driving of motor vehicles by women, etc.), though not government controls on religious expression.

The government places restrictions on religious freedom. Foreigners attempting to acquire Saudi Arabian nationality must either already be Muslim or convert to Islam. Proselytizing/promotion of any non-Islamic religion is forbidden per a 2022 law.

Islam, specifically Sunni Islam of the Hanbali school, is the state religion in Saudi Arabia. According to official statistics, in 2022 90% of Saudi Arabia's 15 to 20 million citizens were Sunni Muslims, 10-12% are Shia, many Twelver Shia populating the eastern regions and Zaydi Shia in the south of the country. More than 30% of the population was made up of foreign workers who are predominantly but not entirely Muslim.

According to a number of sources, only a minority of Saudis consider themselves Wahhabis, although according to other sources, the Wahhabi affiliation is up to 40%, making it a very dominant minority, at the very least using a native population of 17 million based on "2008–09 estimates". A 2014 survey found that 97% of the young Saudis consider Islam "as the main influence that shapes their identity."

Public worship and proselytising by non-Muslims, including the distribution of non-Muslim religious materials (such as the Bible), is illegal in Saudi Arabia. Non-Muslim foreigners attempting to acquire Saudi Arabian nationality must convert to Islam.

The kingdom is called the "home of Islam"; it is where Prophet Muhammad lived and died, and united and ruled the Arabian Peninsula, and the location of the cities of Mecca and Medina, now the two holiest cities of Islam, (the official title of the King of Saudi Arabia is "Custodian of the Two Holy Mosques"—the two being in Mecca and Medina.) Non-Muslims are forbidden from entering the holy cities, (although some Western non-Muslims have been able to enter, disguised as Muslims). The kingdom attracts millions of Muslim Hajj pilgrims annually, and thousands of clerics and students who come from across the Muslim world to study.

In the 18th century, a pact between Islamic preacher Muhammad ibn Abd al-Wahhab and a regional emir, Muhammad bin Saud, brought a revival of Islam (Salafism - that is, following the Quran and Sunnah in light of the interpretation of ‘As Salaf As Salih’) of Sunni Islam first to the Najd region and then to the Arabian Peninsula. Referred to by supporters as "Salafism" and by others as "Wahhabism", this interpretation of Islam became the state religion and interpretation of Islam espoused by Muhammad bin Saud and his successors (the Al Saud family), who eventually created the modern kingdom of Saudi Arabia in 1932. The Saudi government has spent tens of billions of dollars of its petroleum export revenue throughout the Islamic world and elsewhere on building mosques, publishing books, giving scholarships and fellowships, hosting international Islamic organisations, and promoting its form of Islam, sometimes referred to as "petro-Islam".