Conversion to Islam, also known within Islam as reversion, is adopting Islam as a religion or faith. Conversion requires a formal statement of the shahādah, the credo of Islam, whereby the prospective convert must state that "there is none worthy of worship in truth except Allah and Muhammad is the messenger of Allah". Proselytism of the faith is referred to as "dawah", and missionary efforts have been promoted since the dawn of the religion in the 7th century.

Converts to Islam may be referred to as "converts", "reverts", or "new Muslims". Many people who have converted to Islam prefer to call themselves "reverts", in reference to a hadith that says that all people are Muslims at birth, but only come to "leave" the faith due to the environment they are raised in. The belief in the innate condition of Islam in all people is referred to as "fitra".

Converting to Islam requires one to declare the shahādah, the Muslim profession of faith ("there is none worthy of worship except Allah and Muhammad is the messenger of Allah", Classical Arabic: أَشْهَدُ أَن لا إِلٰهَ إلَّا الله و أَشْهَدُ أَنَ مُحَمَّدًا رَسُول الله)

In Islam, circumcision (khitan) is considered a sunnah custom that is not mentioned in the Quran but is mentioned in hadith. The majority of clerical opinions holds that circumcision is not required upon entering the Muslim faith.

Dawah (Arabic: دعوة, lit. 'invitation', Arabic: [ˈdæʕwæh]) is the act of inviting or calling people to embrace Islam. In Islamic theology, the purpose of da‘wah is to invite people, Muslims and non-Muslims, to understand the worship of God as expressed in the Qur'an and the sunnah of Muhammad and to inform them about Muhammad.

Dawah as the "Call towards God" is the means by which Muhammad began spreading the message of the Quran to mankind. After Muhammad, his followers and the Muslim community assumed responsibility for it. They convey the message of the Qur'an by providing information on why and how the Qur'an preaches monotheism.

Counting the number of converts to a religion is difficult, because some national censuses ask people about their religion, but they do not ask if they have converted to their present faith, and, in some countries, legal and social consequences make conversion difficult, such as the death sentence for leaving Islam in some Muslim countries. Statistical data on conversion to and from Islam are scarce. An expert on Islamic law, M. Cherif Bassiouni, states "The Quran contains a provision that says ‘he who has embraced Islam and then abandons it will receive punishment in hell after Judgment Day'." According to a study published in 2011 by Pew Research, what little information is available suggests that religious conversion has no net impact on the global Muslim population as the number of people who convert to Islam is roughly similar to those who leave Islam. According to another study published on 2015 by Pew research center, Islam is expected to experience a modest gain of 3.22 million adherents through religious conversion between 2010 and 2050, although this modest impact will make Islam, compared with other religions, the second largest religion in terms of net gains through religious conversion after religiously unaffiliated, which is expected to have the largest net gains through religious conversion.

According to The New York Times, an estimated 25% of American Muslims are converts. In Britain, around 6,000 people convert to Islam per year and, according to a June 2000 article in the British Muslims Monthly Survey, the majority of new Muslim converts in Britain were women. According to The Huffington Post, "Though exact numbers are difficult to tally, observers estimate that as many as 20,000 Americans convert to Islam annually." In the Philippines, approximately 220,000 people converted to Islam in 2011, known as Balik Islam. The number is increasing year by year, and a surge in conversions is expected in the coming years.